...and even that wasn't uncommon, as far as I remember. I think it was customary that apprentices signed with their master's name in the middle ages and were trained to do just that either to say "that's the school I come from" or probably simply to get all the work done that his master was paid for. Like oil painting the spoilt brats of some aristocrat or something.

I think it's a matter of tradition and what customers expect. If they knew about it and expected no different, that was ok then. It's different today.