isnt crossposting against the rules
It's rule 8, but it's a bit ambiguous.
It says not to post the same thing in different forums (read: boards).
But that's not saying what qualifies, whether it's topics or posts. Nor whether it includes reposting someone elses topic/post. Nor if it applies to someone posting the same thing multiple times in the same board (is 5 identical posts in Off Topic a case of cross-posting, spamming, or both?).
Plus, I have to say I've never seen the rule enforced. The rate of cross-posting itself is low, and almost any examples of bans for it fall into the above murkiness of spam vs cross-posting.
None of this is to condone or endorse cross-posting though. But rather to say it's tricky to question whether someone has broken the rule as it isn't clearcut.
The only perfect example is someone posting the same topic in the likes of Gen Disc, Gallery, Help, Off Topic and Creativity. But even then the only examples of that are Self Delete spammers who are banned more so for spamming and/or posting gross content than for cross-posting.