Most everyone has seen stock spam in their inbox. It is the plain text stuff that sneaks by the filters, telling you about how this or that company is about to explode in value. You check the ticker, only to find it is now on the pink sheets (and usually moved there just a few days before you recieved the spam).
The scammers know the word is out. So what do they do instead of relying on suckers to pump up their holdings so they can dump? They approach the trading companies, offering to pump up their stock in return for a fee.
I guess part of this is playing of the greed of a company CEO who went public with a dream, only to watch said dream go spiraling down the tube. The scammers risk being reported for their approaches as well, but I doubt they care too much.