[rebuilder]

Why can't they just leave the key in the glove compartment? That way, Zipcar only needs to unlock the car remotely when the user starts the rental, and lock the car remotely when the rental ends. In between, the user can use the key. like with every other car.

[technofiend]

Probably to prevent accidentally taking the key. If you've ever used a zipcar, the key is affixed to the steering column. I assume they have an ignition interrupter that requires a proper unlock signal because otherwise stealing one would be matter of jimmying a lock or breaking a window.