Assuming that the electronic key card is programmed until the end of the reservation, wouldn't it be rather inconvenient to leave the room thereafter? The front desk would have to reissue a key?

And one could easily have checked in using a low limit credit card so, unless the hotel processes a charge each day to confirm the guest is still "paying", it would be rather easy to run up a large bill that never gets paid, before absconding in the middle of the night or something...

Luckily most of our society prefer dreaming up such schemes on the internet rather than actually doing it!