> The difference being that a client can be malicious, while e.g. a local file is assumed to behave with the same intent as another.

I'm not sure what it means to assume something about the behavior of a file, presumably thought of as a static piece of data, but I'd certainly disagree that a modern computing system is entitled to assume that all local apps behave with the same intent as one another (except to the extent that it assumes that all local apps behave maliciously).