I don't think it's theft but I also don't think it's really equivalent to that either. After all 1,000,000 people can smell what the restaurant is cooking for another customer and it doesn't cost the restaurant any more than if 0 people did because the restaurant is letting the scent go out either way. On the other hand if 1,000,000 people watch a 100 MB video and block ads it certainly costs the content provider a lot more than if nobody did.