As I understand it, the BSD licence (which BTW only some elements of Mac OS X have, as happens with some elements of Windows) allows to redistribute binaries, modified or not, provided you keep the copyright notice. In that case, how are Apple products "less closed" than, say, Microsoft's?

Note: I barely use Apple or Microsoft, so the question is NOT rhetorical.

In addition to UNIXgod's comment, I can refer you to:

http://en.wikipedia.org/wiki/OS_X

There you will find:

http://en.wikipedia.org/wiki/Comparison_of_BSD_operating_sys...

"... Apple Inc.'s iOS and Mac OS X, with its Darwin base including a large amount of code derived from FreeBSD."

http://en.wikipedia.org/wiki/Darwin_(operating_system)

"Darwin is an open source POSIX-compliant computer operating system released by Apple Inc. in 2000."

Yes, Apple has used free BSD code to build an OS that is not open source. I don't think anyone would dispute that.

Referring to the Berkeley TCP/IP stack?

Indeed. How does using the stack makes windows any less closed? Since you can't even tell to which degree it was modified, I think the answer is "it doesn't at all". And I think the same reasoning goes for mac OS X.