The answer is that he only plays 3 jobs and he thinks that should be plenty for everyone else. There for it must be law.

I'm pretty sure he's afraid that some guy is gonna come along and PWN him on PUP and be like "Hah! I wrecked you, and this is only my 9th favorite job!".

So he figures that if people can't get good stuff for their (9th favorite PUP) because they used all the merits on sam or war or monk etc. then he will always be a big fish in a small pond.