Earlier quoted context omitted.
Exchanging child pornography is illegal in the United States (as well as in many other countries). There's nothing unreasonable about stating in your TOS that you can't have a subreddit the very existence of which breaks the law. Sure, people can try to reopen these subreddits under different names, but once the rule is in the TOS, they can be reported on and shut down as a matter of routine. The same applies to ware…
What are you talking about? Reddit had a very strong stance aginst CP already. And there's a lot of rules broken on /r/trees so it's far from safe. Like meetups to exchange or people giving tips. That's not legal in US and happens quite often there. Same with /r/atheism, a lot of their content can be labeled hate speech under US law. (IANAL)
There are (dubiously) "hate crime" laws, but they pertain to violent crime.
In a similar vein, I'm not so sure about your legal analysis about "arranging meetups" and "providing tips" on marijuana forums. The reason mj forums don't want people arranging meetups probably has more to do with not creating an easy venue for sting operations.
(Disclosure: it's sad it took Reddit this long to apply this rule, and more than a little repulsive to see people up in arms about it.)