No opinion on the shops decision, but I've had the same situation. There are shops that will not fill tanks made of a certain alloy or of a certain age, based on a history of failures. What can happen is that a tank will pass hydro, and said tank will fail eddy current testing after hydro due to stress cracking at the neck. I've had my tanks refused after personally testing them, after going through formal training, etc.I know the tanks are OK, but the shop wont fill em. it's policy. I respect that, Simply because I know that many tanks are not inspected to the degree possible. The shop has no way of knowing, so they make a blanket decision. fair enough.
It should be in our minds that it's not liability per se, but the possibilty of the tank monkey getting their legs blown off during a fill which causes much of this type of caution.
Proud tank monkey,
Nomad