Is it bad for your car to leave the AC on while the car is parked?

The A/C will not operate if the engine isn't running. It's illegal in the UK to leave the engine running on an unattended car.

I've commonly seen people in the US leave their car running while they go into a restaurant. It's idiotic.

It is bad for cars to idle very long. Turn it off and eat in another place.

No, as long as the windows are shut and the engine is running.

No.

If you turn the engine off, the air conditioning stops too. It's a REALLY BAD idea to leave the car running when parked. That's just an invitation for the bad guys to steal your car.

If the engine's running, it's fine. If it's not, you'll lose battery power VERY quickly.

Bad? No. If the temperature demands you need to run your AC use it.

Engine running ?? will warm the engine up a bit, but not harmful................

Why would that be bad? In what way?

If you are trying to conserve gas, turn the engine off. Then the A.C. will eventually drain your rechargeable battery.