I'd argue that what you call "trust-by-default" is an aspect of social intelligence that also enables a lot of very productive relationships. It has its downsides, of course, but I think you'd find that by removing that part of our psychology you'd end up with a miserable excuse for a society.
Agreed; Grice's Maxims/The Cooperative Principle suggest that communication would not even be possible without these kinds of generous assumptions. And I don't think it's getting too quacky to extend that premise to society in general. Since it's so expensive to verify true intentions, when it's even possible at all (which, generally at least, in language, it's not), we have to sort of get by hoping that most of the time most people are being mostly honest in order to successfully communicate.
Of course, we all flout these maxims daily, to varying degrees, from white lies to outright perjury. But viewed as a Tragedy of the Commons, there are people who will advance themselves by deliberately and consistently flouting them, to their own calculated advantage.
