I was confused about what was going on with all these theme shops and how they made money versus what was happening on ThemeForest and this one sentence explains it from a 2009 post.

Straight from the GPL:
b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.