There's tons of risk. Did you know a TON of email servers and spam blockers click all the links in emails and analyze what is linked as well? How do you know you won't be charged for those?
As one of the co-founders, I can tell you that click fraud and bots are certainly issues with many ad networks. This is something we think about a lot. We try to pre-empt this by writing algorithms to not count clicks of known-bots. But, people are always coming up with new bots, so it's impossible to apriori block every bot. So, we are constantly retro-actively checking clicks to make sure they are legit. For example, if a publisher yields more clicks on a given campaign than we predict, we'll retroactively take a look through a series of methodical checks and in many cases will do a series of manual checks. And if we find a bot or a ring of folks trying fishy things, we'll retro-actively rectify the situation, so the advertiser doesn't pay for those clicks.