BSD is a permissive license that allows sometime to take the code and update/change it as they see fit. There are no requirements to publish those changes, and I believe you can license it under a different license.

GPL on the other hand does require that you publish your changes, and that those changes also be licensed under GPL. Thus also requires that a company does due diligence and states what pieces of code they used were GPL and give credit

So in removing the last bit of GPL code, someone can take FreeBSD and do whatever they want without having to worry about what pieces of code might fall under different licenses.