> It is an odd clause certainly, since it doesn't forbid anything that isn't already illegal.
Sure it does. The OS X license agreement contains all kinds of requirements that are unenforceable in a range of jurisdictions. It's not merely a reminder of obligations you already have, it's an explicit limitation of the acts you're permitted to perform with the code.
> it is not part of the licence
It's part of the conditions of use of the source code, so it's part of the license you received the code under even if it's not part of the APSL.
> Free software isn't about the right to break existing law.
So it'd be fine for a license to forbid use by dissidents seeking to overthrow a despotic government? Obviously they're breaking the law anyway and so copyright terms are unlikely to be their biggest concern, but it seems like an unfortunate scenario for licenses to get involved with.