[apenguin]

If this[1] is what you're referring to, it's because the acknowledgements would make things needlessly wordy.

[1]: https://www.gnu.org/philosophy/bsd.html

[kjs3]

So it's verbosity alone that makes "GNU/Linux" not just acceptable but morally required but "This product includes software developed by the University of California, Berkeley and its contributors." just beyond the pale. That explains a lot about RMS.