There is trust and there is visibility. Here’s an alternative example I actually do quite often: I attach debuggers and such to system processes. Not because I don’t trust them to not do something malicious, but knowing what they are doing is always useful to me. If Mail is randomly reading files from my Documents folder, perhaps something is wrong with it. Maybe I should just tell it that I can’t look there and see why it might be doing so. These are things that give me more control over my system, not things I engage in because of a lack of trust.

Yes, but as a user, I expect the OS to behave in a transparent manner. If the OS provides a firewall API, I expect it to send all traffic through firewalls that use that API, not selectively redirect traffic from certain apps or domains.