They are. But the ads on most websites are "provided" by a monetization service that lets websites make a bit of money off the traffic through their site. The scam and intrusive ads are usually 'packaged' to look like a legitimate ad to the computers that serve them out to the various websites. There's usually an algorithm that gets smarter at preventing things like that from getting through as more and more complaints get registered. And eventually a website using this ad revenue service will get fed up and turn them off.
For as long as this particular intrusive ad has been plaguing DisBoards, I'm surprised they haven't commented out the code that lets these ads in.