It's also perhaps more informative to consider what a specific culture or faith believes.
In Barovian culture, all magic is bad. It's all demons. It's twisting your children into caliban. No distinction between warlocks and wizards and druids. Burn the witch at the stake.
In Borcan culture, arcane magic is the fruit of the diabolic match. Again, there's no distinction: all wizards are warlocks. They all get their powers from a deal with a devil. The Church makes a big frowny face. While it's not necessarily evil, it's hella suspicious, and generally believed that the devils will probably get the better of you in the end.
Back when I was with the Vallaki Ezrites, it was broadly accepted dogma that magic comes from either Ezra, or the Mists of Death - and therefore probably from fiendish or unholy sources. Clerics of gods other than Ezra are probably conduits for the power of a demon, so for example, it's spiritually unhealthy to be healed or raised by Lizuca. If you let a random Outskirts cleric cast on you, consider mentioning that in confession and receiving penance for it. It was also cited that that magic is known to twist the unborn into caliban ... so how can it be anything other than unholy and tainted?
My own Ezrite, of Her Third Revelation, and a wizard, had a different and more scientific outlook. Nevertheless, she was strongly encouraged to swear off magic for good. Because by casting it, you're making a pact with fiendish powers, and thereby tainting your soul. After all, Her First Revelation is the Borcan church. After an enormous amount of really amazing RP and self-doubt, she moved to Dementlieu, where magic is considered both trifling entertainment and a crusty academic discipline. It's not satan, it's nerd stuff. But she was also told IC that the Dementlieuse See had to remain in step with Borca, and so couldn't just outright say that wizards are fine. Just keep a low profile.
The Faith of the Lawgiver has a similar outlook: either you're a cleric of the Lawgiver whose powers flow from his divine command, or you're channeling the great adversary Mytteri and going to the Hell of Slaves when you die for spitting in god's eye. Just like the Ezrites above, faithful of the Lawgiver are strongly discouraged from letting non-Lawgiver clerics cast on them. Though in Hazlan, there's an awkward theological tension between that and the fact that both the native culture and the righteous tyrant Hazlik seem to think magic is good actually, because magic is awesome power and might makes right. And that justification, if not the conclusion, is something the Lawgiver can get behind. The dissonance inherent in the situation can give rise to interesting RP.
Consider how all of the above might react to a warlock, both in the sense of how the most surface-level features would appear, and what they would think if they were presented with the sourcebook entry. Each Gazetteer entry has a section about what, exactly each domain thinks about magic. But the archetype of the warlock - as a dabbler who sold his soul for power - is an enduring cultural force in the Land of Mists.
EDIT:
tldr; this isn't so much "don't persecute the poor innocent warlocks" as " ... and throw the wizards on the pyre too while you're at it"