It's not about the ad's being "safe" it's about how much you pay them. They have TONS of shady networks that pay the fee because it's ROI effective to do it.
It's a genius business model with almost no costs at all.
Adblock in my experience is roughly 10-15% of all internet traffic. This is based on real stats, impressions go up this much when you get around it.
Conversely for advertisers, the traffic generally is almost worthless & generates very little revenue at all. But the Ad network's don't care because the majority of Ad networks operate on a CPM basis. It's dirty.