It's never been the case that you have to leave it unlocked. But if it's locked, they will cut the locks off if they need to get inside the bag.
I did MIA-LHR on Sunday night and left the padlock on and the luggage strap around the bag. They cut the lock off - no damage to the bag - and either had a tool to turn the "locking" device on the clip on the bag strap, or were somehow able to do whatever searching they needed to do without removing the strap. They also left me the usual billet doux. Nothing appeared to have gone missing.
The BA terminal at JFK has a nice system if you are entitled to use premium class check-in. There is an X-ray machine opposite the check-in desks. If you'd like to leave your bags locked, you stay with them until they have been through the machine. If they need to look inside, you are there to unlock them. If they don't, you can go and the bags are carted off to the aircraft. Nothing like that at MIA, though.