The linker doesn't sign every binary - it signs every binary it links - i.e. binaries you build on the machine.

That is not the same as it signing "every binary", otherwise there would be no point in signing in the first place.

I'm not sure how I feel about it, personally. I think code signing is a great idea. I just don't believe this single authority approach is the way to do it. I totally understand that as you already trust apple to provide the OS, it makes sense that you can trust them to sign apps - but that shouldn't mean you cant (as a user) choose to trust other parties.

Of course, they don't actually check the apps aren't malicious - they don't even check that you are a real person when you create a dev account... But they do have a signature they can revoke if your software is exposed as malware.