This Euronet "scam" has been around for almost a decade. And they try to scare the ATM cardholder who chooses the local currency conversion by warning them that it is not a set conversion rate. Whenever a friend or family member travels to Europe, I always warn them about it. As for Euronet's proliferation at airports and train stations, how is this much different from Travelex being the only ATM choice (that I can tell) at LHR? Where they want to charge you a fee for withdrawing money. And BTW, I have seen this fixed home currency conversion scheme at regular bank ATMs outside the Eurozone as well. Not all, but some.
Oh... and another BTW.
This will happen at your hotel and retail store as well. Always insist upon the local currency for the transaction.