If you click on a link just to say that you clicked on it, then that could be considered spam. Spam can be anything from unsolicited e-mails to bogus clicks on banners. This is not good. It may not be illegal, but it can be traced and you could get your membership revoked. In this case spam can ruin the credibility of a PTS service and the pay will plummet, and in some cases there are lawsuits.
Lawsuits are only possible because spamming is against membership agreements.
Fulfilling membership agreements. Thou shall not spam is the biggest one. People just don't like it. More details when you join. Surfing for money.