All the time on NulledBB, at least once a day I see a spammer. I really do not understand why people feel this need to spam, as they can just contribute, or respond to other people. I understand that they are frustrated by the 15 post limitation, however that is just a minor barrier. In my case, I make at least 15 posts on NulledBB daily, depending on whether I have time or not. Also, if those people REALLY don't want to write 15 posts then they can just buy an upgrade to prime for $10 to get past it. Spamming is really bad in general and I feel that if you spam just to unlock a few posts, then you obviously deserve a ban as you are a leecher.
Mainly people may do it for expressing their anger or to get attention. People may also do to get entertained. You can report these accounts if they are targeting you or someone.