They're also on Time Soldiers cabs. Maybe Ikari, Guerrilla War too. Not sure. I played a lot of Time Soldiers in the arcade. I don't like them things.
Since there is only 8 directions to aim the gun the solution would be simple to convert to a twin stick control scheme. Don't need to change or recode the original ROM either. Just add some code that translates the control input into the proper amount of turns. Since machines are faster and more accurate than humans, this could even make the game control easier and more responsive if applied well.
EDIT: They went with this because there was gun and grenade each had their own fire buttton. Twin sticks + shoulders = solution.