Probably not a question of warranty but rather fulfillment of contract which Sling have failed to do by simply dropping off a parcel on your porch. They owe delivery to either yourself in person or someone in your household / neighbour who would be deemed responsible to receive the delivery on your behalf. Too bad for Sling if on this occasion their courier staff have messed up. Thus, Sling are obliged to a renewed delivery on which you should insist immediately.
Hope your new Slingbox arrives soon!
It is truly unfortunate that someone stole the Slingbox replacement from your door, but this is not in any way the fault of Sling, no more than if you left your door open and someone stole it from inside your house.
Sling turned the delivery of your warranty replacement in a timely basis to either UPS or Fed Ex, normally completing their responsibility. If your experience with Sling was similar to mine, they forwarded shipment tracking information so that I would know when to expect my package, ensuring someone would be available to accept delivery. They also offered to ship my repair to my office, where it could be securely delivered. Their customer service was timely and courteous.
Your anger is vented towards Sling because your package was stolen by someone in your community, how can you blame Sling for that? Sling Media may choose to give you a free replacement, and this would be most kind of them if they do, but that would be at their discretion. I do not agree with you that Sling is at fault.