And, why not? macOS is Apple’s IP and they have all rights to do with it as they want. Buy the way, Chrome/Node.js JavaScript engine uses JIT compilation too. Are they affected?
It is not obvious to me that this breaks POSIX compatibility. The kernel may choose to signal a process with SIGSEGV on a memory protection violation but I can't find anything that suggests this is required.
Last I checked, macOS formally maintains POSIX certification. Linux is not POSIX compliant, so I wouldn't use Linux as the measure of what is correct behavior under POSIX.
Maybe I'm being pedantic but as far as I can tell that document doesn't say when a SIGKILL can or can't be issued. So it seems like it would be valid to issue a SIGKILL at any time the kernel wants. Obviously, that's probably not users/programmers want but it seems to technically meet the specification you list.
If that document said something along the lines of "a SIGKILL may only be issued when XYZ" where XYZ didn't include this case then I'd agree with you. I don't see anything in there that says when a SIGKILL should be or, more importantly, should not be generated. So it seems perfectly valid for an implementation of an OOB memory access to generate a SIGKILL and a SIGSEGV. That SIGSEGV will never be seen because the SIGKILL can't be handled and will kill the process.
Ah yes that is fairly clear, thanks, I was not able to find that. A plausible interpretation is that the kernel still reserves the right to terminate the process at anytime, including immediately after a general protection fault. Still an unexpected behavior, much like the Linux OOM killer.