WARNING: Someone standing can still initiate combat with YOU if YOU are the only one sitting (and you will fight like you normally would, though I recommend standing up).
I understand that in real life, fighting someone while you are sitting down is possible, especially if using a ranged weapon. However, do to our action system, many things are limited when you are sitting, such as FLEE, GRAPPLE, etc.
This fix is mainly to prevent someone from initiating combat in a vehicle, because then there is no way to stand, flee, exit, etc.