PushButton Engine is absolutely free. Modify, release, rewrite whatever you need to get your game shipped. The whole thing is available under the liberal MIT license.
We believe that basic engine functionality is a commodity. You don’t pay for your logging system or your memory manager or your file formats. Why should you pay for your game engine?
Spend your money on what matters for your game. Make it worthwhile for someone to write a great particle system, or a physics editor, or awesome AI. That’s what the PushButton Engine and the component store is all about.