I felt this way as well (since the mid nineties) until a couple of weeks ago when I read _Social Architecture_ by Pieter Hintjens. It has a strong idealogical flavour. He stresses that you should focus on building a community, with the code and platform as happy side-effects of that community.

For that reason, you want GPL because it forces everything back into the community. He gives examples of a friend of his who slaved on a BSD project that got forked with the friend having nothing to show for it. And NT adoption of much of the code from the BSD sockets API.

"This software is distributed under the terms of the GNU General Public License version 3 (GPLv3). Minoca offers alternative licensing choices for sale. Contact info@minocacorp.com if you or your company are interested in licensing this software under alternate terms."

https://github.com/minoca/os/blob/master/LICENSE

Basically, the old MySQL business model.

It looks like the developers want people modifying their code to contribute back, either by giving back the changes or by paying them (they offer an option to purchase a more permissively-licensed version).

Indeed. GPL makes sense in the short term, but in the long run an permissive licensed project forces its branches to contribute back to avoid divergence. GPL makes it harder for commercial entities to invest into it and also a permissive licensed project makes a bigger competition to e.g. Linux.