Well it seems that forums with a fair amount of members are the ones being targeted.. Then again maybe they just scroll through and pick them out randomly.
No, they are bots and don't look at the member counts at all. The guild forums aren't exactly high in members. Not by forum standards, anyway. The (obviously unmaintained) PSMC forum also has rather few members, none of which have posted anywhere near recently, yet it's swamped by spam (even though the only ones "reading" the spam are the other spambots, so they end up replying to each other).
Basically, they're programs that harvest forum addresses from the web (via HTML links), just like email spiders. Then the address list is fed into a spambot that knows how to register and navigate the forum software in question (possibly via plugins for easy extending). In the end, the only person forced to spend time on that are the forum mods / admins that must remove that junk, while the spammers collect the money.
However, the only reason why there still is money to be made with spam is because people keep following the links in it.
