In theory you are correct he who owns the pump has last say unless you want to really get involved with a battle. Those business’ that require a license and insurance are required to follow fair business practices and not discriminate against anyone in any way. They are also required to comply with CGA and DOT regulations. So in effect you could go after their business license if they refuse to fill a tank that is within compliance. Then it gets even more technical as they could claim upon external inspection of the tank they found reason to require another VIS or Hydro. It becomes a pissing contest at best. If it were my tank, in compliance and I didn’t have the ability to fill it myself I would probably just politely remind the owner that filling your tank brings you to his shop for your other diving necessities but if forced to invest in another tank before CGA/DOT regulations require it may cause you not to spend that money with that shop and instead cause you to take your business elsewhere. Even if he is the only fill station in town you can threaten to purchase your own pump and compete for business instead of being taken advantage of.