It's game theory. If you charge money for something, it gives you an incentive to avoid doing sneaky things that will destroy that money stream in the future, like embedding spyware in your app. But if you don't, people wonder what the hell's in it for you. And they figure that since you're not making money off customers, you must have some spyware or adware deal to make it elsewhere.

Game theory's probably the most depressing and cynical academic subject that I've ever studied, but it does do a remarkably good job at describing people's actual behavior.