That is currently not the case, and I don't think it should be. If I write a piece of software and release it under the GPL, I can sell it as proprietary code

That is currently not the case, and I don't think it should be. If I write a piece of software and release it under the GPL, I can sell it as proprietary code as well, dual-licensed. I am in no way obligated to provide updates and changes under the GPL too.