I dont have much experience with contributing to commercial pay-to-download gpl themes, but I have experienced that bug reports, comments on shortcomings and enhancement proposals, and template code contribution haven`t been very welcomed. However this may be because of business models where volunteers do support for free to push customization services.

I also believe that you dont have to be a coder to contribute because sometimes its SEO , usability or documentation that needs changes.

Some of the pay-to-download gpl themes services are not really open-source in the traditional way, its just a marketing adjustment to fit in with WordPress/Automattic policy. Hopefully more will move over to the free to download - pay for support business model, focusing on customer support, addon-packs with value, and quality development, rather than on marketing and affiliating schemes.

A key to success in any business is to listen to your customers, and a business model where customers may contribute in shaping the product will probably give higher brand loyalty, with more referrals and purchases of add-on packs.

Anyways - theme developers are free to do whatever they feel like with their businesses - just my two cents.