From a strictly legal perspective I suspect /bin/true (or at least AT&Ts implementation) do not live up to the law's requirements of originality and expressiveness.
Considering that they were the copyright holders for unix at the time, and that this 'default interpreter processing'/ return code mechanism was part of the kernel/userland interface, I'm not so sure that's the case..
My guess is that the copyright is for the whole body of work, and not just for the single file. The copyright notice in the single file is probably not a requirement from a legal standpoint and is merely indicative.
