Yes, that's true. Who knows why they weren't charged for other crimes. But, in any case, under the law, they faced a maximum sentence of seven years for this one crime. They were sentenced to two years - far less than the maximum.
I can't find the group's position on hate crime legislation. To be honest, in searching for it, some of the websites are just too vile. But, considering the group's left-wing politics - according to Wikipedia, they oppose abortion restrictions and support "LGBT" issues - it wouldn't be surprising if they themselves support hate crime legislation (oh, the irony of it all).
Anyway, why are people so supportive of these women? Is it because they're "girls" - more specifically young women in their 20's? They probably look harmless as they skip away from the security guards at the cathedral. Here's a website with more info on what they were singing about and the words they were using.
Suppose these were men doing the same thing? Would they have come across as so harmless? Would so many people be as supportive? Consider how the women were walking up to police women to forcibly kiss them. Would men get away with doing the same thing? We might not have even heard the story if not for the name - PR.