If 0% of them reply they make money.
All they get paid for is to send the ads out to as many people as possible. It's the companies whose motives should be questioned.
And all this kind of stuff increases the company profile. If you keep seeing "Free sex-vids on DailyRedHeads.com" 500 times a day in your Inbox, when you begin to talk about your disgust at the spammers you will say "Man, I hate all this DailyRedHeads.com crap" and the person to whom you are talking might actually care enough to visit/sign-up.
See, I just did it in that last paragraph. And I'm betting at least one person goes and tries to access that domain/site. So it obviously works.