I can agree that the rule does need tweaking.
In my opinion, the rule is worded like that to prevent people from trying to rob others in cars because it's so easy to disable a car with a single gun shot (not realistic at all). If the rules were flat changed to allow a robber to shoot at someone in a car, then there is 0 opportunity for someone to legitimately escape if cars are disabled so quickly. As soon as the gun comes out the other party has no option and the "hands up rob" mentality will only get worse.
To combat this, I think we would see civilians (and some criminals) fleeing for their lives as soon as they see anyone remotely getting near them (does this already happen?). I understand Los Santos is a hyper violent city but closing this "loophole" will further make civilian life worse and they shouldnt have to be watching their back 24/7 even in the pseudo safety of a vehicle. I dont want to prohibit people from robbing others but chain crimes/robberies and broad daylight/public area robberies are low RP but not necessarily NonRP.
On the other side, Fear RP rules state just some examples, not all. If someone is completely boxed in and has no where to flee, it's time to give up whether your car is stalled or not.