Activation lock has absolutely nothing to do with a carrier lock.
The activation lock is an anti-theft measure that can ONLY be turned on by the owner of the phone. It is the owner's responsibility to disable the activation lock when they transfer ownership of the device.
Activation lock does NOT lock the device to a company or carrier. It locks it to the owner of the device.
Apple can not and will not intervene unless the individual possessing the phone can prove that they are the original purchaser of the device.
YOU are responsible for making sure the used device you're purchasing is actually usable.
If it is activation locked, and the individual you purchased it from can't or won't remove the lock, or you are completely unable to contact them, it's useless and there is a very strong chance that the device is stolen.
Turn it over to the police.