I'm the author of an eCommerce plugin hosted on the WordPress repository. It's specific to the First Data Payeezy credit card gateway. I also sell a premium plugin that adds functionality to the basic plugin. Frankly, it's functionality most users don't want or need.

I sell the premium plugin under GPL 3. What I'm really selling is my expertise. What I'm selling is not code, but premium access to me. When someone buys my premium plugin, they get my personal cell phone number and email address.

If someone wants to give my premium plugin away for free, they're more than welcome to do so. There's nothing unethical about it. I just won't provide free support.