All the repositories are GPL'd, so I'm assuming they meant build in a purely educational setting.
It's a nice move, but why not go all the way and let developers leverage your code in their proprietary code? Insurgent Games only open sourced once they gave up, so they clearly know that open sourcing all your code is not a good idea when you're selling games.
License
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This game is licensed under the GNU General Public License (see gpl.txt). As the copyright owner, I hereby give anyone permission to re-license my GPL code under a non-GPL license for the purpose of distributing it in the iOS App Store or the Android Market.To be a bit pedantic, all you can really say is that they didn't know whether open sourcing your code is a good idea when you're selling games. I could imagine myself being convinced that having open sourced a game makes you no more or less likely to lose market share to copycats than keeping it closed. I don't know that anyone's tried it, nor that there's a particularly good experiment one could do to test that.
Um, how about in a hobbyist setting?
Only a small subset of the people who write games are going to make money doing so. Many of them know this and do it just because they enjoy it. (See: Almost any mod community.)