I trust the claims because Googles business depends so heavily on PR. If Google leaks everyone's browsing history, they as a company will cease to exist.

Notice how there are lots of people claiming 'Google collects XYZ info', but no claims of 'Google zipped up XYZ info and sold it'. Every article about Google selling user data is in reality Google using the user data for it's advertising business, but explicitly not letting the partners see the data - as well as being legally protected private data, that data is also a competitive advantage they would lose if they handed it over.

Less public facing companies on the other hand I don't trust, because they don't have much to loose on the PR front. That miscellaneous credit card processor? Yeah - they'll probably be handing your data over to every credit check/profiling agency.