I wonder if this is a design "feature" of that iphone model? When the engineers sit down and design the hardware and software, somewhere along the line a decision as to what gets shut down when airplane mode is implemented has to be made---and someone decided to shut down GPS also. GPS is a passive receiver: no RF emissions, so it does not need to be shut down to comply with airline regs. But someone might have decided to play it on the safe side.

My Samsung android does keep GPS operating in Airplane mode, but nothing says all phones have to implement Airplane mode in exactly the same way.