Original sentence: In addition, it was a large legal problem for those wishing to use BSD-licensed software under the GPL: the advertising clause is incompatible with the GPL, which does not allow the addition of restrictions beyond those it already imposes
Edited sentence: A large legal problem was that the advertising clause was incompatible with the terms of the GPL (which does not allow the addition of restrictions beyond those it already imposes) thus forcing a segregation of GNU and BSD software.The GNU project  suggests people not use the phrase "BSD-style" licensing when they wanted to refer to an example of a non-copyleft license, in order to prevent inadvertent usage of the original BSD license