Regarding the "risk" of filling unfamiliar steel cylinders, it would make ALOT more sense to me if there was even the tiniest bit of documented proof that someone has been injured filling steel tanks at a scuba shop. 6351 AL tanks, sure, I would understand that, because there are cases of those tanks failing. But, if you can't tell the difference between a steel and AL cylinder, and you don't know the most basic facts about filling these tanks, then you have NO BUSINESS 'professionally' filling cylinders. Being so ignorant about your business is a FAR great risk than filling an old steel tank.
The combination of ignorance and arrogance exhibited by some LDSs regarding fills is the issue. And of course, it's the shop's compressor, they hav the right to refuse to fill any tank they want. Nobody's going to argue that, but why do they have be uninformed about it? You'd think that they would actually want to learn something about compressed air cylinders and would want to better understand the risks, or lack thereof.