Good question but I don't know.

Apple did not create A/UX. It was created by UniSoft

https://en.wikipedia.org/wiki/UniSoft

https://virtuallyfun.com/2021/09/19/so-what-is-the-deal-with...

I'm guessing that Apple would have to pay a fee to both UniSoft and AT&T and maybe Sun was much larger and a partner with AT&T / Unix System Labs on SysV so maybe they could resell Solaris for cheaper.

Sun open sourced Solaris in 2008 for 2 years. Maybe AT&T / USL didn't care by then or maybe Sun's licensing agreement allowed them to do it. I don't know.

A/UX was also ridiculously insecure in its default configuration. It had a "guest" user, no password. I remember labs of Mac II or IIx systems at a local university, with public IP addresses, guest accounts, no passwords. A local BBS guy posted a message on how to get "free" internet access through the university's dialups. I think it was at least a month before things got locked down.

It was the time, only trustworthy people on the net so far. Told the story here about my mid-ninties job where I ran Quake servers on the lan before we had a firewall.

Yes, though contemporary OSes like SunOS were more secure than A/UX. Once you were on the network though, it was like swiss cheese (NIS / YP... unshadowed password files... etc.)

Interesting what-ifs. Perhaps offering the money they spent on Copeland to ATT could have bulk licensed it.