There's no reason for a limited speed, assuming flight assist is off.
The reasoning that the thrusters are programmed to stay within a limit doesn't fly, because after shutting off the thrusters while descending towards a planet, a ship should keep gaining speed at a constant rate until impact, yet it is still staying limited.
The reasoning that the thrusters are programmed to stay within a limit doesn't fly, because after shutting off the thrusters while descending towards a planet, a ship should keep gaining speed at a constant rate until impact, yet it is still staying limited.