Of course they are liars. There is no other way to describit. When the card is first swiped, assuming it is not an emv card, a message comes up on the terminal asking if the customers wishes to be scammed oops wishes to allow dcc. Now if you're saying that low paid clerks with no loyalty to the outfit are simply taught to press the accept button, I won't argue with you. But every pos terminal has to have the ability to avoid the transaction (what if the clerk enters the wrong amount (accidentally or on purpose)...One time in Holland I got that story and insisted the cashier (she was a waiter) walk through the transaction step by step with me (after voiding the original one) and by and large as soon as the card was swiped, indeed a message came up asking if I wished to pay in US dollars or euro. She told me she had been instructed to press US dollars as a courtesy to the customer (of course she has no idea what a scam this is, she probably has never travelled out of the euro zone in her life) so I told her to press the euro button. She did and voila, the transaction went through in euro only properly.

While we can argue just how much it pays to argue the point at the merchant (yes most of the time the low paid help doesn't have a bloody clue) I always at least make an effort and then go through the marking of the sales slip with dcc declined etc. Now whether it's usually a waste of time, I won't dispute. But I don't like being lied to.