Here are a couple of tidbits as someone who ships UPS a lot. If the value is over $200 they require a direct signature, you can't even leave a note for the driver. What this means is either the driver bypassed the rules and it's on him/UPS or the retailer didn't declare the value of the package (ie they didn't insure it) and it is on the retailer. Either way it isn't on you, the only way the retailer has proof you actually took possession of the goods is your signature on the UPS scanner and clearly if tracking says front door they don't have that.
My guess is that the retailer is trying to save on shipping by not paying for insurance...which is fine but it puts it on them if you don't get it. Their other option is to check a box when shipping that says to require adult signature, that doesn't cost them a dime so there is no reason they can't/didn't do it.