That's always been one of the ways to make money from free software. Mysql did the same thing, and I think Qt did as well at some point in its history. If I recall correctly, even RMS sort of approves of it.
Downvoters: go read some history, there's nothing factually incorrect in what I wrote above. The bozo proliferation seems to continue unabated here:-(
Here's RMS himself: http://www.fsf.org/blogs/rms/selling-exceptions