Eh. I don't think that many people actually have multiple accounts. I've only known three, including myself, and my second one isn't even active right now (and only even got started so I could get that stupid mount since I still haven't managed to get the stupid sandstone drake recipe). Additionally, they'd make tons of money from people "compressing" accounts since it costs something like $25 per character to move them between accounts.

I don't know what their reasoning is, but I seriously doubt it has anything to do with people paying for multiple accounts. There's probably money somewhere in the line, but that revenue stream is probably too small to be a part of it.