Red Hat Enterprise Linux is open source software, yet people still pay for that.
You're purchasing a support contract, from Red Hat. You're not buying the software, you're buying their consultation. Similar to IBM and Linux.
My GPL is a bit rusty, and I'm not familiar with the problem that has been cited with the dev allegedly circumventing the GPL.
There is nothing wrong with charging for an application that has been released under the GPL. It's one of the top questions the GPL receives.
If the guy is evading releasing his source, for the front end that communicates with that transmission library, then no doubt that's sleazy.