Most people don't run an adblocker on their phone, and these torrent sites would cease to exist or become malware sites themselves if enough users used adblock.
Let's not kid ourselves- these site runners are not Aaron Swartz.
So what? If you are looking for an epub file, but instead get an exe just don’t run it. Mobile OSs are quite good at security, but even desktop has largely grown up to the task now.
It's not reasonable to expect site operators police every ad if they use a major ad network, like Adsense. There are millions of creatives, and new ones get uploaded every minute.
Hope that you didn't click a sketchy ad that looks just like the download link, because the site didn't police any of its ads.