No need to wonder, Mikestang, I'll tell you.
Accounts are being farmed by budding bitcoin entrepreneurs. These accounts have to make a certain number of posts, over a certain period of time, before they become "Full Member Accounts."
Needles to say, account farmers generate some volume.

Once these accounts are ready for the market, they are sold, right on this forum, in Auctions or Digital Goods sections, to their proud new owners.
"Why would anyone buy an account, when they could make one for free?" you must be asking right about now. Lolno, you're not. You know why it's done - to make money from ad campaigns. Yeah, those farmed accounts just keep on giving.

So now you don't have to wonder about where all the spam comes from - it's bought and paid for