[carlosjobim]

If a browser blocks the website owner's ads and put their own ads instead, then they are leeching on the work of other people and also profitting from it.

There is a saying "You can't fool an honest man", and it holds true for this situation as well. If the browser would just block legitimate ads and put their own ads and take all the money themselves, then hackers would riot. But since they say they are sharing the profits with users, many people will turn a blind eye to such unethical behaviour. Because people are base.