This is true. However, while jokes are not expressly against the rules, the community moderation rarely favors them. Personally, I read a lot of Slashdot, and to some degree enjoy the level of humor, political debate, and anonymous commentary there. I find that the signal to noise ratio here is somewhat better. I don't think it likely that anyone would be banned for exclusively making joke comments, but generally I think the expectation is that while informative or insightful commentary may also be humorous, the primary purpose here is not entertainment.