It's due to the risk of infection.
Once you water is gone the baby isn't in it's sterile little bubble anymore so theoretically nasties can get in making both baby and you sick.
I do know someone who's baby was stillborn when this happened, very sad - she was left with broken waters for a few days and an infection got in
Although I think 12 hours is a bit OTT, I would be nervous being left for as long as three days. If that happens don't have sex or go putting anything in there either for that matter