Why "progressives" so stubbornly cling to such a benign view of Islam is unclear. I suppose it's part wishful thinking or a result of humanist sentiment, or perhaps due to the influence of writers who assure us that Islamic doctrine is, in fact, peaceful.
Yet, if one is to judge Islam by its works, the picture that emerges is anything but comforting. Historically, Islam has persecuted non-Muslims virtually each and every time it has held a plurality. Where Muslims dominate(d), but did not persecute others, was generally a result of colonial or dictatorial powers that held them in check. There are few exceptions. It is a fact which stands to this very day. Even in Turkey, which is often held up as a positive example, non-Muslims have lived a tentative existence (and not that long ago suffered horrifically under the Turkomans). In fact, recent events in Turkey see it sliding ever more into the arms of Muslim fundamentalists.
Another "moderate" Muslim realm so often held up as an example of Muslim "peacefulness" is Indonesia. True, Indonesian Islam from the time of the Dutch East Indies, tended toward a live and let live Islam that blended local animism and Muslim tenants. It is interesting to note though, that even during the colonial era, fundamentalist Muslim insurgencies were known for the there murderous ways and zealotry. As in Turkey, modern Indonesia is witness to a slow inexorable slide toward fundamentalism and its accompanying human rights abuses.
Essentially, it is a general rule that whenever Islam becomes the dominant religion, non-Muslims become targets of persecution. Finding exceptions to this rule is a challenge.
There may be any number of reasons why Islam is so uncompromisingly supremacist, but one reason stands out among all others. That is the fact that the foundational texts of Islam are supremacist in the extreme ... to the point of advocating violence as the main form of enforcement. To read about Muhammad, whose example all Muslims are required to follow, is to read about a brutal chauvinist who used murder, deceit, theft and all kind of barbaric means to pursue his agenda. Furthermore, he took the chauvinism common in his time and expanded on it, making Christian chauvinism of the era pale in comparison.
Compare this to Christianity. Anyone at all familiar with Christian foundational texts (culminating in the New Testament), understands immediately that Christianity is by and large a pacifist religion. Christian foundational texts focus on spirituality and are almost completely devoid of political guidance. Christian foundations can best be described as "turn the other cheek". Muslim foundations can best be described as "smite the unbelievers."
To understand the difference between the two, one must understand the historic context in which each came into being.
Christianity came into being under the reality of Imperial Rome. Early Christians could not conceive of life apart from Roman dominance. As a result, the religion offers virtually nothing in the way of political instruction. Literally, it leaves the "sorting out" to God in the afterlife. For example, the ordering of punishment for "sin" is a matter to be dealt with between the sinner and God when the sinner dies; very little earthly “legal” remedy is offered. Roman law was so over arching, that to suggest any form of material reward, sanction, or other, would have been to challenge Roman law and order. Furthermore, there was no chance, even remotely, of expanding Christianity using the sword ... so early Christians expanded the religion using the persuasion of ideas, good works, and a message that promised eternal life based on belief in Christ. Early Christians focused on the example of their prophet, Jesus Christ, who set about as pacifist an example as could be. His primary tenet was, "the greatest of these is love". His most violent act, the driving out of money changers from the temple.
Contrast this with early Islam, where the founder initially had no powerful and dominating Roman empire to contend with. Muhammad and later Muslims were able to expanded their faith solely via the sword. After a fragile start, Muhammad plundered his way to dominance. Muslims became the masters of their destiny and their religion became law where they lived. Muhammad died ruler of his realm; a realm that expanded bloody year after bloody year. As a result, Islam needed not be pacifist, it needed not to moderate, it needed not to compromise. It moved far beyond spiritual matters and became a way of governance, of economy, of war, of life, and death. Islam, became a road map for all aspects of life and governance ... today we call it Sharia.
Critics will no doubt recall the violent excesses of the Church of Rome, and they will be correct in doing so. Christianity, despite its pacifist foundations, has been used by political forces over centuries to enact all sorts of violence. Once it became the dominant religion in Southern Europe, it's pacifist nature was quickly circumvented for any number of reasons, but none of these reasons could either then or now be justified using foundational Christian texts. As a result, Christians have always struggled back to their pacifist and peaceful roots. Why? Because their foundational texts are such, and they find more than ample reason there for coexisting peacefully with others. In fact, it is a great theological stretch for Christians to march to war. When they do, they do so largely because their foundational texts deal almost exclusively with spiritual matters and don't cross over into governance, offering little guidance on political matters. This fact is critical to our discussion.
Moderate Muslim reformers have no such reality. There is no "peaceful" Islam to return to. Muslims who value modern notions of human rights and liberty must work counter to their prophet, they must, in fact, reject his example. They have no "Jesus" to follow. Their prophet is a barbaric sadist. Their early texts are a prescription for human rights abuses, for war, and for utter and total domination of others. There early deeds are conquest, pillage, and the subjugation of all their neighbours. Islam was born of war and violence. Since there is no peaceful Islam for reformers to go back to, it can be argued that truly devout Muslims are those we Westerners wrongly call radicals ( al quaida, ISIS), for they seek the supremacist Islam of Muhammad and are truly not radicals at all; while in reality the actual "radicals" are those who seek to reform Islam into something more compatible with Western ideals of liberty and freedom. Islam that conforms to Western notions of human liberty is radical; Islam that is violently supremacist is traditional.
That is why Muslim apostates like Wafa Sultan, Hirsi Ali, and others, argue for "transformation". They understand from experience and education that there is no peaceful Islam to go back to ... the concept of "peaceful Islam" in the Western sense of "peace", is a myth, a construct of ignorant Westerners, deceitful Muslims, or worse yet, delusional but well-meaning secular Muslims who after 1400 years still can't admit to the violent core of their faith. It's time that Westerners realize that while individual Muslims may be peace loving, their religion is anything but.