I believe it is because if one attacked a Reform "jew", their intent is to actually attack a Jew. The fact that the one they picked isn't actually a Jew makes no difference. It's like firing bullets into a picture of a Jew when you thought there was an actual Jew standing there. You meant to kill a Jew. It is also true that if the Reform "jew" had a Jewish mother or maternal Jewish grandmother, they are actually a Jew and it is to be fervently hoped that they will find their way back to a Torah-true lifestyle and take up Judaism the way it was meant to be observed. Therefore, they are defended.
Oh come one, we're talking about calling people out for advocating and working to make happen an end to Christian morality, not smacking someone in the head with a brick or anything like it.
I won't stand and agree with a good friend who is off on a tangent that's destructive of the moral good just because they're my relative but it's Ok for people to accept and defend destructive efforts of others because someone, somewhere, may want to hit someone of like background and heritage with a brick one of these days?
I understand what you're saying but that's pretty much granting the people working to destroy the social fabric of this country a permanent,"din do nuffin" card.