They don't think about it. They get paid to post their links EVERYWHERE online. If even a very small percentage get clicked, it might generate revenue for the person who hired them to post. They might make money with the scam or by infecting computers with a payload.
Don't think for a second that some guy in Pakistan, China, Russia or wherever is actually picking and choosing places to post links. They are hired by companies and governments to get people to click on links. And they do that by spreading the links en masse.