Quite common unfortunately. It means they get very little profit compared to selling you the tyres and fitting them. Some fitters (like Graham's) take it as an opportunity they'd normally miss and welcome the small bit of work, but a lot of bigger places that have a good long queue anyway, can afford to refuse hoping you then buy from them. You'll be surprised as to how many fall for it, as if they didnt, the fitter couldnt get away with it
Micheldever Tyres (Winchester) are very good and will take tyres from Camskill and fit them, or with Black Circle you can get them delivered direct ready for fitting. They do an excellent job and are the only fitters I take my cars to. I've never had anything mis-balanced from them and they always nip the nuts up with torque wrenches, not air guns.