I'm aware that Notch isn't in charge of the forums... not even Minecraft if you want to debate that. Still, considering there isn't a reputation system on here. It seems kind of odd that theres a limit to the likes. I mean on a single post it makes sense.
It occurs to stop spammers that try to increase someone's reputation by going into all his posts in order to get him popular. If there wasn't that message the spammers would 1+ every post that made someone.