DY – I see your objection, but there are three factors here:
1. Labeling something as avodah zarah is pretty serious, something that needs a posek or a talmid chacham. Just as we say that someone is a tinok she’nishba if they grow up without any yiddishkeit at all, so you could say the same about someone who is raised with a certain derech that most of us don’t accept, people who honestly believe that they are practicing authentic yiddishkeit.
2. Declaring certain groups “beyond the pale” has halachic consequences. We can’t eat their shchita or drink their wine. But there are also yichus problems, similar to the ones faced by some Jews from India or Ethiopia – maybe their marriages are valid but their gittin aren’t, leading to sofek mamzerus for the whole group. And these are mostly people that we couldn’t invoke the idea that sofek being a Jew cancels the sofek of being a mamzer. No matter how confused, members of these groups are still recognized as yidden. So what happens when the third generation wants to rejoin the mainstream?
3. My mother AH used to say, “You catch more flies with honey than with vinegar.” Ridiculing people is absolutely guaranteed to make them stop listening to you at all, when if you speak a little more softly, you might have a chance of your tochacha being heard. דברי חכמים בנחת נשמעים
That said, there are groups whose shchita I do not eat and whose wine I do not drink.