Hotels charging in USD instead of stated currency

My bill for a stay at a Doubletree hotel in Europe was in Euros, yet the hotel charged my credit card in USD rather than EUR. The conversion cost was 5%. I was never notified either before or after checkout that this would happen. (I didn't even check it until another issue came up, as there was a deposit that was charged directly in EUR, so it never occurred to me that they might charge the balance differently without asking.)

I wonder if Hilton has any policy on this? If not, maybe we should have a list of hotels known to rip guests off in this way?