Without trying to take too much away from your arguement, your router's antennas are relatively low-power, low-amplitude units that BROADCAST, whereas your phone is RECEIVING a very-high amplitude broadcast from an outside antenna.
Anyway, cellular signal is meant to be able to go through basic environmental object such as a few walls or a couple trees (which is why you can call/text people inside a house). Based on this alone I don't see why the iPhone's antenna wouldn't be able to receive cellular signal if it is partially covered (with the "death" grip). So I would definitely call this a design flaw on Apple's part for using "cheap" hardware/engineering. I don't want to take away from the enginuity of the company as a whole of course, but for this particular piece of hardware they have failed....with multiple cellular providers.