As far as I can tell, it is an US thing. I can't speak for other countries in the Americas, but when I was in Europe briefly a few years ago, what few purchases I made with a card were all done chip & pin.

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.