This is equally true for almost any game ever sold, including physical ones. You only ever own a license that specifies what you can and cannot do with the game. The difference is in what this license is tied to, for example either a physical copy of a given game or an account that can be remotely deactivated taking away all your games. In GOG’s case once you grab the installer, the game license cannot be easily forcibly revoked, just as with the physical copy.