PRWeb also has a problem with people using them for spam. For example the press release you see below was republished on sfgate.com. Once published it was pushed into SFgate's Most Read list by bots.
At $159 per press release it's probably cheaper than other spam vectors..
Paul Lindner So, was that done by PRWeb itself or by spammers using PRWeb?
Kevin Cox, Nov 30 2012 on 1500wordmtu.com