Apple could agree to any software licenses they wish, but they choose not to with GPLv3. There is nothing legally that prevent them from giving users access to source code, nor giving users permission to change software running on devices that they have bought from Apple.

Apple refuse to do this. No one know for sure but the common suspicion is that they want to avoid competition by locking users to a single platform where apple has a artificial created monopoly.

Or maybe they just don't want to give away one of their core products for free.

Which core product would that be? GPL require that you give out source code of derivative works (work based on someone else work).

One could be accused to think that the core products of Apple is the devices that they sells. What clause in GPLv3 require that they give devices out for free?

yeah, best not use free software if you get the willies from the word "free".

Does that affect regular OS X desktops/laptops? It makes sense for Apple TVs and iOS devices (well, as much sense as locking them down in the first place) but for OS X, the anti-TiVoization clause merely requires that the end user be able to replace those binaries. And there's always a way to turn off System Integrity Protection.

Or are they worried about something else, like the patent clauses?

The patent clauses. They're so vague and untested a lot of corporate lawyers balk at having anything to do with them.

What is the difference between GPL2 and GPL3 that makes this impossible from a legal POV?

I don't think anybody knows whether there is a real difference, but the increased strictness of GPL3 such as its anti-TIVOization (https://en.m.wikipedia.org/wiki/Tivoization) clauses certainly make a lot of lawyers concerned.

Where exactly the border lies between and not linking is a grey area, legally, that hasn't become clearer with GPL3.

It wouldn't surprise me to see bash go from the standard install, too. Apple is slowly removing all GPL code from its OS.

I hope they make ZSH the standard shell. It's similar enough to bash to not disrupt most workflows and Apple has been able to update it due to its permissive license.

I use ZSH as my main shell on OS X. I find it superior to bash in almost every way (especially with oh-my-zsh) except in ubiquity. Even so, I have no problem using bash if that's all that's available, the two shells are fairly compatible.

It was the primary shell in 10.2 or 10.3 after tcsh or csh (its been a while I can't remember well).

if someone uses your software in a way that is honoring the GPLv3, but you don't like, you lose any ability to sue or further restrict them. Even though the ghost of Steve Jobs (Forstall) is no longer haunting the building, Apple learned the lesson of "sue early, sue often", and Stallman/Moglen sort of got in the way of that with GPLv3.

Tivoization was another issue, especially with Apple's tendency to lock down their platform.