It does make absolute sense
There are two pockets bhind the seat that can be used to hold things (flat things). If you have the top open and the car locked, you don't want someone to be able to just reach in and move the seat forward and grab whats in those pockets. Thats why you can only move the seats with the doo ropen (meaning you have the key to unlock the door) or with the ignition on and the doors closed. Similar to the reason why they made the central locking button deactivated once the engine is turned off, so that it prevents someone from reaching in and unlock the doors and alarm when you park with the top down.