When you ask "why did big company X make strange choice Y regarding licensing or IP", 99 times out of 100 the answer is "lawyers". If the Plan 9 group had had its way, Plan 9 would have been released for free under a trivial MIT-like license (the one used for other pieces of code, like the one true awk) in 2003 instead of creating the Lucent Public License. Or in 2000 instead of creating the "Plan 9 License". Or in 1995 instead of as a $350 book+CD that came with a license for use by an entire "organization". Or in 1992 instead of being a limited academic release.

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".

I'm also curious why they picked the GPL, but why on earth is it a "shame"?

If you don't want to see a proprietary fork of your code, you should use GPL. If you don't mind seeing proprietary forks of your code, you can use BSD, Apache, MIT or any other more "permissive" license.

I don't think Plan9 has much commercial value in it left, but they may consider it would be awkward if, say, EMC or Cisco incorporated some of its technologies (Plan 9 had some interesting ideas about storage and network transparency) into their proprietary products without paying for them. It's easy to sympathize with the small garage inventor and to think it's nice for you to subsidize small innovators, but it's much harder to do the same with corporations that would patent-extort you the moment they realized they could gain some advantage from it.

To forbid getting money leeching the work of others for free.

If you look at the note on the home page[1], they needed to get permission from Alcatel-Lucent for the license change. They may not have been able to sell them on BSD.

http://akaros.cs.berkeley.edu/akaros-web/news.php

It does feel a bit odd when it is the B of the BSD license.

Please see my other comment, title is misleading: https://news.ycombinator.com/item?id=7232450

Plan 9 continues to be LPL.

This isn't a shame, it does nothing. Plan 9 was already released under a BSD style license (one with an absurd amount of pointless legalese, but non-copyleft nonetheless). This is simply releasing it with additional restrictions so that people who are contributing to GPL software can use it now too.