Perhaps "balloon grade" here is not "party balloon grade". Weather balloons? Research balloons?

My guess is that places like AirGas aren't really supplying many weather or research balloons. I suspect the easier answer is 'Balloon Time is low grade crap aimed at people who don't know any better and just want to pick up some balloon gas while grocery shopping.' It's like the difference between people who go to a gas station to refill propane tanks, and people who swap them at Home Depot. (though the smart fellers do swap at Home Depot occasionally, if they need a fresher tank...)

Definitely worth knowing what you're getting, in any case, so you don't get ripped off, and so you can actually get that lawn chair contraption into the sky.

AirGas prioritizes industrial users, in the case of helium, copper welding. Argon is perfectly good enough for almost all welding purposes, but copper is different because of its heat conductivity. The heat from the weld really wants to go anywhere else. Helium has substantially higher heat conductivity than argon, which allows the heat to flow from the electric arc into the metal faster, resulting in better welds.

Obviously you can't have oxygen in welding gas; it would oxidize the shit out of everything.

A little bit of oxygen in party balloon gas is beneficial. Some kid will breathe it, and when they do, you didn't want them to asphyxiate themselves.

Do you know if the other 20% is oxygen (as was claimed) or if it's air? I just think the latter seems cheaper and more likely.

I think it may be CO2. CO2 in the gas would cause all sorts of unpleasant effects that would discourage continuing to breath it, and CO2 is probably cheaper to store and transport than oxygen.