Why? I can see some legal responsibilities coming from that. A lot of shops want the owner to check their gas before they leave and acknowledge the MOD involved. That way the diver is responsible if he/she winds up dead from breaking the rules. Its sad it has to be that way. If it wasn't for that I would agree with you.
I suspect this may be more of an American tort law thing. Over here in the Netherlands it's far more relaxed - I've never even been asked for my Nitrox c-card to get fills. The place I normally use doesn't even have a log for you to sign.