I don't believe they have in any way violated the letter or even spirit of the BSD license. It's extremely liberal, and should be.
However, they used to really flaunt their BSD roots on OSX, but seem to do very little to give back in the form of code, money, or even PR for that matter.
Here's an example of that:
https://developer.apple.com/library/content/documentation/Da...
But now they've even removed any mention of this, and allowed projects like OpenDarwin and such to just plain die.
I think it's pretty poor behavior from a company that's SO involved in intellectual property rights to just take something as valuable as a complete kernel and userland (not GUI) and then just sweep it under the carpet because no laws require them to do otherwise.
Puts a really bad taste in my mouth to see such important work treated like that. I'd have expected it from IBM, but not from Apple.