The problem is that, if the term "GNU" is preferred, we might as well open doors for two things that can make the term not a good choice:
* Some non-free system distributions are also GNU, so we wouldn't be able to call just/only the free ones as "GNU". * We might end up advertising non-free system distributions indirectly, even though we didn't mention them to the general public. I have seem a person that is still studying the free software philosophical movement, and he thought that Raspberry Pi was a good choice of single board computer just because some "GNU" system can be used, then I told him that only the non-free system distributions can be used on Raspberry Pi, and pointed him to the page on FSF's website about the single board computers.
