Since *BSD isn't encumbered by the GPL, people can (in principle) make proprietary software out of it and make $GINORMOUS profits. Thus following the model:
1) Take BSD OS or software.
2) Modify it, making unique, closed-source solution.
3) ???
4) $GINORMOUS Profit!
One can't do #2 and distribute the result (the GPL forbids it).
However, since there isn't (AFAIK) any BSD software that doesn't have a GPL work-alike, there's the issue of competing with free software. So it's not a very successful business model (the exception being OS X which, as you note, is unique to Apple's hardware. But, of course, the BSD-ness is only in the Darwin kernel.).
Cheers,
Scott.