Yep, and they charge you for a new one.
What I think it is:
They don't know on a return if the tank is any good or not, so they have to test it to be sure (they do that on exchanges too, but that's an exchange, the whole point of the business).
So it might just be sold for scrap, or it might end up being reused. They don't know. So they "just take it".
That's making $$$ in the end, on average.
They COULD just give some amount of cash. But then they'd get "tank scavengers" collecting 50 year old tanks and returning the crap for cash that would just necessitate a bunch of testing followed by scrapping. And up their potential liability risks. So they don't. They just "take them".
As far as the cashier's knowledge, well, every place has procedures that MUST be followed. If you or I were on the other end of the transaction, we'd want to make sure we "did it right".
And as usual, sucks to be you. See patch avi.