It's a matter of trust...
How much will you put up with to get something for free? And even if you're willing to put up with annoyances yourself to get things for free, how willing are you to send other people to those same annoyances?
On the web, many people simply don't care. They'll link to sites which use pop-under ads or other overly-intrusive ad tactics without thinking twice. Of course, they can blame it on their pop-up blocker and say they didn't know (which is why I don't use a pop-up blocker – it would prevent me from linking responsibly).
If you're not annoyed by such intrusions, more power to you, but you may be due for a reality check. When someone repeatedly sends me to such annoyances, all I can do is walk away and not come back. Good riddance.