Signatures? Is this an America-thing? Me and my MasterCards go back sort of a long way, pens and paper never involved.
In the US, that signature is a on a line right next to the words "I agree to be bound by the cardholder agreement", or some similar language. If I had to guess, the signature was originally supposed to be a mini-contract that yes, you're going to pay back the card company for what they just paid on your behalf. I don't know if such has ever been been taken to court to be enforced, however.
Because that would be financial fraud for which they can go to jail.