The main difference is that /r/jailbait directly broke reddit rules, was illegal content, etc. While /r/fatpeoplehate followed pretty much all the rules and had strict moderation. Once /r/fatpeoplehate was banned, and other 'hate subs' that were near identical weren't banned, it became clear the admins banned subreddits based on the idea, rather than behavior (contrary to what they claimed). And then things sort of just went to shit. It was sort of the final straw for a lot of people, since the dislike of reddit has been growing for some time now.
This is a popular myth, but it didn't break any rules at the time (which is why there was a decent-sized outrage when it happened; reddit basically ignored their own rules to get rid of a subreddit), and it wasn't illegal - there's a reason the government didn't go after violentacrez once Gawker revealed his real identity and he stupidly went on CNN to defend himself. The subreddit drew a line in the sand and banned anything which crossed that line. It was absolutely immoral and creepy as all fuck, but they deleted everything which crossed the line into being actual child porn. it wouldn't surprise me in the least if some of the moderators were saving the stuff they deleted to their own hard drive (or a server hosted in another country more likely), but what they allowed to stay on that subreddit was within the bounds of legality.The main difference is that /r/jailbait directly broke reddit rules, was illegal content, etc.
As a site though, Reddit shouldn't be asked to keep an eye on that sub to make sure they're not hosting child porn. FPH shouldn't have been banned because it's a stupid little circle jerk and who gives a rat's ass, but jailbait was literally posting pictures of young people who are meant to look as if they are illegal to post pornographic pictures of, and everyone was expected to show deference to that rule on the honor system. The porn industry jumps through a lot of hoops to make sure that everyone is over 18 when they're working (taking video testimony while the actors hold an ID and social security card and verify their age), but jailbait didn't do that and it's not worth the risk. FPH has no inherent risk. It's just a bunch of jerks laughing at fat people. However, if Reddit wants ad-revenue from larger corporations then it's going to have to clean up its shadier subs, and that's what they're trying to do now. First FPH which won't have any defenders, then a few more here and there, and then eventually you have a squeaky clean Reddit. Hubski has a way better system. This whole place could be filled with violent sex-offended racists and I just filter them out. Never see it.