Second, I disagree that it is 'directly against the rules' - no where there does it say specifically that you must not post solutions elsewhere. I think the intent is there - but if someone simply cribs off another answer somewhere, they're not really learning anyway and are really robbing themselves. Project Euler is altruistic anyway and there's really no 'advantage' to stealing answers. (I guess someone could show off their progress, but I don't think that's much of an advantage, since they aren't really learning anything)
I have learned quite a bit from looking at Project Euler answers from others and am glad they published their solutions. For example, there are many different ways to do #2 in Haskell and it is enlightening to see how they work.