There are some things that copyright law allows you to do. If something is licensed with GPL then the license allows you to do things that plain copyright doesn’t allow, as long as you meet the conditions in the GPL license.
I checked the answer you’re referring to. It says that the poster of the question only did things that copyright law allows. And it’s absolutely true, if copyright law allows what you do, then there is no need to care about the GPL license. Your conclusion is wrong: The things you suggested, like copying and selling the software, are not allowed by copyright law, so you must read the GPL license and check whether you can do this, and under which conditions.
And in the USA, the GPL license is a license, not a contract, so you don’t have to agree to the license. You just need to do what it says if you do things not allowed by copyright law. Whether you agree with it or not. Like if you drive and there’s a red traffic light, you don’t have to agree that you have to stop – you just have to stop.