It didn't make itself "deliberately incompatible" with all free licenses before it, for example MIT/X11 is definitely compatible. Why is the original BSD license singled out?
https://www.gnu.org/philosophy/bsd.html
Many people besides you believe proprietary software is harmful enough to enact defense mechanisms when proprietary software vendors harm the user and lock down free software into their products.
Well, we're in a thread in an article about the GPL, what did you expect? :-)
Furthermore, I'd argue that a lot of those defense mechanisms that copyleft supporters claim are necessary cause much more harm to Free Software than they do to proprietary software. The ability to use GCC for static analysis, for example, was muddied for many years by the GCC team's unwillingness to create a suitable API out of fear that it would become easier for proprietary software teams to use GCC without contributing. Meanwhile, proprietary compiler chains, like those Microsoft provided, were easily able to create those same tools because they were not driven by such fears. Additionally, you have statements from people like RMS trying to memory hole software ( http://gcc.gnu.org/ml/gcc/2001-02/msg00895.html ) that doesn't agree with his political views. While the goals may be noble, the route the free software community has chosen to get there has devolved into stubbornness and childishness to the detriment of everyone.
That is irrelevant if DTrace used copyright assignment to ensure that there was always one copyright holder, regardless of the number or affiliations of contributors. I honestly don't know if they did or not, but from what I've heard in this thread it sounds like they did.