I agree the 2000 and 2003 releases' strange custom license terms sound lawyer-driven, but weren't the 1992 and 1995 releases' terms also driven by hopes of commercial exploitation? My impression was that during this period AT&T still hoped they'd be able to license Plan9 for $$$ to a vendor, like they had done with Unix, which meant they wouldn't want to release the code under a permissive license.
Yes, but who tells you "you can't do that because we want to keep open the possibility of making money later"? The lawyers.
AT&T gave away Unix during Unix's formative years, because AT&T's government-granted monopoly disallowed it from making money on computers. They didn't make any money on Unix then. They tried later, and made some, but what a sense of loss about what might have been if only they'd been charging from the beginning! Of course, if they'd been charging from the beginning, it's hard to say Unix would have been so popular.
Thankfully I am not at Lucent anymore and am not privy to the tortured negotiations that ended up at the obviously inelegant compromise of "The University of California, Berkeley, has been authorised by Alcatel-Lucent to release all Plan 9 software previously governed by the Lucent Public License, Version 1.02 under the GNU General Public License, Version 2." But the odds are overwhelming that the one-word answer is "lawyers".