Microsoft launches open source foundation (Linux-Watch)

I think this is the crux of the matter:
Many "BSD people" release code with the (not so) secret intention of getting hired and making money. The code is a means to that end.
Most "GPL people", on the other hand, are (A) forced to release because they are adding to a GPL'd base or (B) because they care about the code and want help from others.
That GPL'd projects enjoy a wider developer base seems a consequence of type A developers, and maybe the fact that it looks much more fair from an occasional contributor perspective (the ones that would not be hired).
And of course, users don't care much as long as the stuff works.