This is thought to be because of GNU Coding Standards: https://www.gnu.org/prep/standards/standards.html#Reading-Non_002dFree-Code https://www.gnu.org/prep/stan

This is thought to be because of GNU Coding Standards: https://www.gnu.org/prep/standards/standards.html#Reading-Non_002dFree-Code https://www.gnu.org/prep/standards/standards.html#Reading-No... They wanted GNU implementers to be careful not to copy (even through their recall) AT&T UNIX implementation, but to do compatible solutions in different way. This allowed them to keep safe footing legally through the time when the original UNIX source code's copyright status wasn't clear. The point of the GNU project is to create independent UNIX compatible operating system. The BSD implementations are often referred in this discussion, but regarding them, you've got to keep in mind that it is continuation of the original UNIX source tree, after it's copyright status was clarified (https://en.wikipedia.org/wiki/UNIX_System_Laboratories,_Inc._v._Berkeley_Software_Design,_Inc. https://en.wikipedia.org/wiki/UNIX_System_Laboratories,_Inc....).