Salesman scam, something for nothing?

A travelling salesman is about to check in at a hotel when he notices a
very charming lady giving him the eye. In a very causal manner he walks
over and speaks to her as though he has known her all his life. After a
few moments both walk back to the check-in-desk and register as Mr. and
Mrs. Smith.
After a three-day stay the salesman walks up to the desk and tells the
clerk that he is checking out. The clerk gives him his bill for £3,000.
"There must be a mistake here!" The salesman protests.
"I’ve been here only three days."
"That’s right" The clerk replies, "But your wife has been here a month."