Yes, the guys wearing white hoods will probably not collectively crap all over the knitting club. That doesn't make the overall situation any better, nor is it a good idea to have them make up a large part of the site's userbase.

(And the only reason it seems that they aren't a problem is because moderators of more specialized reddits have to delete their posts, and ban the posters.)

Some customers aren't worth keeping.

You seem to concede my point, but then proceed with your argument that it's still a problem, anyway. I don't know how to respond to that except to say that I hope nobody actually listens to you. Reddit has some great communities and people shouldn't be scared to participate in them.