Payment terminals can offer whatever currency exchanges they want, but usually it's just a way to fleece you on the spread, nobody is doing you any favors, it's just that whoever in the chain gets to perform the exchange gets to set the fees and the spread and most people get confused by currency exchange so it's in every middle man of the chain's interest to be the one to perform it and get the spread themselves.
If you use your bank's rate by paying in foreign currency then this is usually fair, at most they will add a 1% foreign currency transaction fee.
When it comes to exchanging cash, avoid currency exchanges at places like airports, tourist hotspots, etc. as they will usually offer worse rates than elsewhere.
