Well, for one, you can't make small modifications to the code and then resell it as a commercial product with the original author's copyrights tucked away in an about box. You'd actually have to release the source code too. It will be harder to convince customers you've added value if they can see exactly what you've changed.
We have some ideas on how we'll facilitate that. There will definitely be ways for people to sell plugins they create and or for large companies to keep them closed source.
