Planes frequently push back from the gate on time, but then wait 2 feet away from the gate until it is time to queue up for take-off. This increases fuel consumption, and increases the time that passengers must sit in a cramped plane awaiting take-off. Why does this happen?

How will commercial airlines respond to the threat of new $27,500 fines for keeping passengers on the tarmac for more than three hours? What inefficiency will this create?