I have done some research on this because the liability seems to vary from region to region which actually makes quite a bit of sense. Our relatives in Brooklyn, New York have had UPS and FedEx packages stolen from their front porch three times. In all three cases the merchant, not the shipper, covered the losses and resent the items.
From the perspective of UPS, they stand to be more profitable if they have to eat the costs on the occasional to very rare stolen delivery. This is rather straightforward to see if you consider the fuel costs and vehicle maintenance cost involved with taking, say, 4 attempts to deliver a $40 package. Put another way, if 1 in 10,000 packages delivered are stolen, UPS paying for that is generally cheaper than the aforementioned costs for re-delivery attempts.
UPS does keep the steal percentages by region and have decided from almost an actuarial perspective that dropping packages off at the door step is overall less expensive than administering to the costs and headaches associated with attempting to re-deliver when resident is not at home or does not feel like answering the door.
We have inexpensive home video surveillance systems to thank for this along with vigilant neighborhood watch programs that are quite prevalent and effective!