While CO2 is CO2, you never know whether the people that don't sell it as food grade aren't using the manifolds and other equipment to fill your bottle with that they may have just used to fill other gases with.
Sure, it's probably not enough to really worry about. But as some are picky about a cubic inch of air getting in their fermenter, they may not want a cubic foot of O2 mixed in their CO2 bottle. Which if they don't flush the lines with the new gas before filling your tank might be a real possibility.
Welding supply houses deal with a lot of different gases. Acetylene in beer might be an interesting taste.
If they aren't willing to call it food grade, then they aren't taking the procedures to make certain things don't get mixed up. IMO.