it's not a "fraud always." there are lots of businesses nowadays that have become so reliant on the digital card reader that they don't have a clunky old imprinter. and there are lots of servers who really don't understand how the process of using credit cards works.
even if a restaurant has more than one credit card "machine," it's extremely doubtful they have more than one credit managing service. by "machine" the OP's server was probably refering to the cash register that server was using.
idine restaurants have no way of knowing a particular customer's card is registered, until after the card is run.