It is the dealer trying to get away with saving a few quid. So many people wouldn't even notice. I bought a year old Yaris for my Dad last year and it needed a pair of front tyres. They tried to get away with putting some shit tyres on it, but I insisted that it had Dunlops to match the rear. It was an approved used car, therefore should be to factory spec. It took a lot of arguing, but I got the Dunlops in the end.
In the case of the GTI, they came from the factory with either Dunlop, Bridgestone, Michelin, Continental or Pirelli tyres. You might be able to convince a main dealer to fit the correct tyres, but if it's an independent trader, as long as the tyres are legal, he's doing all that he has to. I would be prepared to walk away from the deal if he won't fit Michelins for you. It's not as if there aren't other GTI's for sale.