As I understand it, if you write a piece of software, you're the one holding the copyright. Therefore you decide under which license you will allow people to us

As I understand it, if you write a piece of software, you're the one holding the copyright. Therefore you decide under which license you will allow people to use your software and whether you want to charge for this use or not (unless the license itself specifically states that you must or must not charge for the use of the software, in which case you must conform to the rules of the license). If you decide that you are going to charge for the use of your software and somebody is using it without paying for it, they are in violation of the license. If you release your software under GPL and decide to charge for it then yes, you have to give the source for free, but only to those people that payed for the license to use the software in the first place.