Other mistake: In this episode, when Kirk's phaser was on overload, it made a small explosion, but in "The Conscience of the King", a phaser on overload would destroy 2-3 decks of the Enterprise.
Other mistake: After Kirk makes the cairn of rocks to bury D'Amato, he places a rock with the man's name on it. If the phasers couldn't cut the rocks of the planet, how did he cut a squared rectangular rock, and how did he put D'Amato's name on it?
Chosen answer: If they're in orbit, they're being pulled along by the planet's gravity well, therefore, impulse engines would only be used for minor corrections and would be "on standby" while in orbit, but not active. (Like keeping your car idling without revving the engine and creating plumes of exhaust).
Captain Defenestrator
Thank you for the info.
Movie Nut