> any service provider needs to have some discretion on these things

Of course, but that's not the issue under discussion.

The issue under discussion is the combination between (a) how vague these things are (b) the extremely wide range of badness (don't tell me that "self-promotion" falls into the same category as "spam/scams") and (c) the fact that your email account can be locked if you violate these rules on the forum.

It's one thing to claim that you're closing someone's forum account because they were harassing someone on it, and another to close their email account. That's not protecting your forum, that's trying to give yourself the ability to legally retaliate against someone for anything that they did that you don't like.