Forgive the obvious question, but why did you need to sign? I’m in the UK and I’m genuinely confused....
Short answer: some of the US merchant payment processors are still in the 20th century.
While we've gone to chip cards mostly, it's all chip+sign not chip+PIN. Idiotic, right? Not sure of why, perhaps the card companies figured people couldn't handle pins even though they do for their debit cards (which I NEVER use).
Some places will still print out a slip and require signature even for ApplePay transactions. Other places don't. It's semi-random and even the transaction amount doesn't seem to make a difference.