For me, because Islam is most definitely objectively evil in its current incarnation.
Other people may be reacting against falsehood and lies even if the doctrine doesn’t have evil consequences for this world, but only for the next one, rooted in an innate love of truth, the finding of the light of the world. Some, experiencing this, curse the darkness instead of lighting a candle. Some do both, lest we forget the darkness is there or grow accustomed to it.
To lapse into Calvinist or Augustinian terminology, those who leave the truth for the darkness, the reprobate, feel doubly much the need to curse the light which they have abandoned, either out of pride, or envy, or hatred, or a con-fusion of the three, so that they might wander blindly while yelling, “I can see”, freezing to death while claiming they’re on fire, lest they find themselves pulled back towards the light, where they might refresh their eyes and warm themselves. Or those with such a great concupiscence that they leave the light so that they might sin, as an attempt to rationalize sinning and hold repentance and contrition at bay.
Those who journey out of light and into darkness often do so for subjective or emotional reasons, not an actual, rational inquiry and subsequent decision (some step into the darkness on their path to forming their knowledge and a rational warrant for belief in the light; some never even know what the light is, being shown only a caricature of it: I digress, as the very rational inquiry I am talking about precludes the caricature being the only knowledge had): rational inquiry in the sphere of religion leads always to Christianity, and eventually to Catholicism, if taken to its ultimate conclusion, but people may give up the search at any point on the path, either tired of the work and inability to commit, or thinking that once they have seen a spark, they are next to the source of light. This is one of the reasons that fideism is so dangerous, and is used by the Devil with such effect: those who are too proud or too lazy either stay in the religion they were raised in with no questions (whether it be true or not, they never learn), or join with whatever is popular at the given hour - today this is the pop-culture, unlearned militant atheism of scientists like Richard Dawkins and second-rate philosophers like Daniel Dennet - for a ready-made set of easy answers without the need to challenge their preconceived, received system of thinking, nor that dominant at the current time in their society.