The honest answer is that people releasing software under the GPL don't want you using their code in your proprietary whatever. They explicitly said they don't want you to do that.
How is that different than you releasing your proprietary whatever, and conditioning use on paying you money and agreeing not to share it? If you get to set the terms of use of your code, so do other people.
> What's the point of open sourcing something if it can't be used in a proprietary product?
Obviously, the point is to share it with people who want to do something other than use it in a proprietary product.
Complaining about free gifts generally doesn't end very well. Don't like the terms? Pretend it is Oracle or Microsoft crown-jewel code. You can't do what you want with that, either.