Releasing it under the GPL allows them to be compensated by the additional contributions made by other. Just because the transaction doesn't involve cash doesn't mean there isn't an exchange of value.
The irony of the GPL is that, much to Stallman's chagrin, it actually reinforces the ideas of intellectual property ownership and fair compensation for the use of other people's work.
“From each according to his ability, and to each according to his need” or something like that?