May I suggest to use the "Archive" function once you download a game with this platform, this will create local files (that you can burn or store) to reinstall the game without having to redownload the whole damn thing.
While I understand your pain, uninstalling a "previous" version is quite often a requirement for applications & games and demos could be understood as "previous" versions.
On the other hand, with a little registry knowledge it's probably fixable. But I agree with you that the game should definetely check out if any demo is installed and suggest to uninstall before continuing ...
I have little hope of this going to be fixed for galciv2 though, since galciv2 won't have anymore major update.