At the firehouse we get packages from UPS fairly regularly... and we hate them. Even if we're at the station they'll just leave the packages outside of the back door, signature required or not. They just don't seem to care.
FedEx always makes sure we get our packages, and won't leave them without a signature. They'll actually go out of their way 3-4 times a day if they don't get us the first couple times because they know many of our packages are time sensitive (AED pads, gloves, things like that).
I used to chat with my UPS guy at work nearly very day (I got 3-4 deliveries every day) The company just ramped up his work load constantly. His route ran 4 times longer in a little over 3 years and he had to pay his own parking tickets.
He used to be great, a real part of my business, but he burned out very quickly. The last few years I had a new guy every couple of months.
He told me that the "no signature" thing was a liability issue. By ringing the doorbell, they can't just give the package to whoever answers. They're obligated to make sure that it reaches the required place. IE if they hand it to a thief, they're liable. If they put it down (as per instructions) and a thief then takes it, then it's not their liability.