There's a perverse incentive there. If I'm paid to watch adverts, and my love for money outweighs my hatred for adverts, then I'll watch lots of adverts without actually buying anything. That's great for the ad-provider, but not so much for whoever's products are being advertised, as they'll be getting a significantly worse return on their investment. It's actually in their interests for me to block adverts if I won't buy anything that's advertised in them.