They aren't direct employees of the team or stadium. Wonder what the source of the decision to charge for tap water was?

If they didn't charge for it, some manager may have told them after-the-fact that they should have, and docked their pay.

Why were they getting it at the concession stands?

I don't recall seeing one (I've honestly never gone looking for one). A person could always fill a cup from a bathroom sink if they really wanted to.

"It is the first time that I have ever heard that complaint here" maybe it's the first time they tried that shit.

You could refund any credit/debit purchases in theory if there is a tracking of what was purchased provided the vendors didn't just lump all purchases of water under bottled water, but that would leave out people who paid cash.

The best they can really do is to get an estimate of what was charged and donate that amount. There's really not a good way to make it right for the people in the stadium.

Fix it.