If I were in your shoes, I would analyze it like this; Tank is plus rated, but they refuse to honor it. So you are paying full price to fill a 72 but getting shorted 7cf on every fill (steel 72 at 2250 is about 65cf). The top ups on the HP tank is free... would you feel bad for getting topped up after using to do gear checks before a trip? I would not feel bad about using to get the full fill you PAID for. If your 72s were not plus rated, the ethical math works out differently. I'm in the market for a new Hydro facility, due to the my LDS uses not doing plus ratings. So my LDS only fills my 72s to 2250.... I usually go elsewhere so I can get 2800 in them, but when I use the LDS for it I have no heartburn. There policy matches what the tanks are currently stamped for. I also see no ethical concern in renting an AL80 to transfill my 72s higher with... If I bought the air in the AL80, it's mine to use as I see fit. If your HP tanks came with free fills for X amount of time.... you bought that air (with the purchase of the tank). Draining the tank for no reason to get refilled would be a.... rude.... choice, but even that would be within bounds... just really not nice!We actually have free fills on the hp100 for another year, but the shop is 50 minutes away. I am also not sure this would be the most honest thing to do.
Of course, my advice is worth what you paid for it!
Respectfully,
James