Must be regional. Whether a signature is required is up to the shipper, and Amazon generally doesn't request one. You could follow a UPS driver around in the US and pick up boxes from in front of doors all day. There are 1700 YouTube videos called "UPS package thief".
In Germany it's normal that you have to sign for your packages and I'm not aware that there is an option not to do that.
That being said, some delivery people simply dump the package in front of my flat and shipment tracking says I (who wasn't home) signed for it. And of course the fun of living in a dorm with 120 people and the illegible signature (are there others?) and you don't know who it belongs to (because the slip of paper in your mailbox just says "delivered to neighbour").
I don't think the handover issue is a fundamental problem with the drones though. For example you could receive an automated phone call to let you know the package is hovering above your house. If you're ordering something to be delivered in 30 minutes you're probably going to be there to receive it.
But I still see theft in transit as an issue. The moral/mental threshold is surely much lower for shooting down a mechanical fly than it is for raiding a van.
The threshold for destroying a device costing several thousand dollar which is actively having its location monitored?
If a police car is nearby it could be on the scene before the drone even hits the ground and having a few stories of parents getting a giant bill might do the trick.
Also I would imagine that the drone could be transmitting a live video feed. Anyone trying to steal/attack the drone would be recorded trying to do so.
