Selling software via a proprietary license to an enterprise can be bad for open source because it means that the enterprise doesn't need to obey the terms of the GPL. They are free to modify and adapt the code without contributing the changes back to the community, which kind of ruuing the point of the GPL in the first place.

This method also only works if the copyright if fully owned by a single person or entity, with all code contributors signing a contributor licensing agreement to give away their copyrights to a single entity. This is problematic because now there is the possibility that this single entity gets bought out by an evil corporation (say, Oracle), which can choose to continue developing the project without releasing the modifications under an open source license.