[Lorkki]

> There is a risk that simply including or using bash4+ could infect the system with GPLv3 compliance requirements, forcing Apple to publish their proprietary source code.

This seems dubious, since the FSF themselves make the statement that aggregates of differently licensed software are fine, as long as you comply with GPL for the GPL-licensed parts: https://www.gnu.org/licenses/gpl-faq.en.html#MereAggregation

[floatingatoll]

> But if the semantics of the communication are intimate enough, exchanging complex internal data structures, that too could be a basis to consider the two parts as combined into a larger program.

All of macOS, since 10.0, is built on ever-more-interlocking systems of ‘intimate’ communication. As upthread points out, there’s a hardware chip that uses IPC to interlock with SIP to protect system files. The depth of IPC used to deliver a Macintosh appears to qualify the entire shipping OS for the “larger program” clause.

Setting IPC aside, it’s still easy to construct a case against GPLv3: If bash is included in an operating system release that can be downloaded like any other program on the Mac App Store, then it’s absolutely plausible to a layperson that the inclusion of GPLv3 in that program would - like any other program - infect it in its entirety with GPLv3’s copyleft requirements. macOS isn’t an aggregate software repository to its users, and a judge could easily be convinced to agree.

I’m not Apple’s lawyer, but I hope this helps convey what I imagine is part of their reasoning against it.

[Lorkki]

That's really not different at all from any other operating system. The "laypeople" in court have previously been perfectly able to consider even highly integrated OS components as separate from the whole - such as Internet Explorer in the case of the big old Microsoft antitrust lawsuit.

I think that a more likely reason is that Apple have plans for macOS that wouldn't play well with GPLv3's anti-Tivoisation measures.