Why hasn't the creator been sued into oblivion?
I think it's because technically they aren't sold as "Calvin" peeing on something. It's just a cartoon drawing of a boy who happens to resemble a more famous cartoon character. Unless it actually says "Calvin" they can probably get away with it. Remember the Kimba/Simba lawsuit?
There isn't a single creator. They're bootlegged by a lot of different people that have small presses, and sold independently. Since Calvin's not marketed with the dolls, stickers, etc., it doesn't really hurt anyone economically, so there's not a lot of incentive for the legal copyright owners to try and shut them down. There have been a few cases where Calvin stickers have been confiscated from auto shops, but it's been half-hearted enforcement, at best.