There are two things that should be note about copyright, which would have prevented AT&T or anyone else from actually enforcing any copyright on such simple files.
First, copyright does not protect ideas--it only protects the way an author expressed the idea. The closer the expression is to the minimal way to express the idea, the less likely it is to be copyrightable. In the case of the simple implementations of /bin/true and /bin/false, they are pretty darned close to minimal.
Second, if you have author X who produces a copyrightable work, and author Y who later independently produces a work that happens to be identical to X's work, but Y did not copy elements from X, Y is not infringing X's copyright.
This second case doesn't happen often. If someone were to decide to write a novel about boy wizards, and produced something identical to the first Harry Potter book, no one would believe that they did this without copying from Rowling. However, if the work is small enough, an identical independent work is believable.
In the case of the simple /bin/true and /bin/false, a defense that you didn't copy from AT&T would be believable. Even if you admitted you looked at the AT&T source or had it described to you, it would be believable that you produced your own expression of the idea of "a do nothing shell script" and a "shell script that exits with status 255".