i can't believe how bent out of shape people get when the light ahead of us is yellow or red and we pause to let someone exit a parking lot and squeeze into our lane. we appreciate it so much when someone lets us do it so we like to pay it forward.
Because that action can cause accidents.
At a red light in general you are not supposed to block access points so you should leave it clear. Cars turning have the duty to ensure they can do so safely. And it's a huge no no to try and cross multiple lanes or for people to try and turn left into a business (or street) even if traffic is stopped as generally one or more lanes still has cars that can move. That leads to t-bone accidents a lot. People
think they are being nice by waving cars through since they didn't block the access point but unless all lanes are clear it can easily cause an accident.
On a yellow light if you suddenly stop
just to let a car in a parking lot turn you can create a traffic hinderance and should not do so. You become the unsafe driver.
The same goes for when the light is green which is even more egregious of an action in terms of safety.
It's not the same as letting someone pass (as in you're both walking in) in front of you while you're both entering a store where you might think "oh well that was nice of them". You are in control of a vehicle and sudden actions can cause unfortunate situations.
Yellow lights, if I read straight from my state's driver's handbook, states
"Steady YELLOW Traffic Light – This means the traffic light is about to change to red. You should slowdown and come to a complete stop if traffic flow to the rear allows. If you are already in the intersection when the yellow light comes on, you may continue safely through the intersection." But don't arbitrarily slow down just to let a car exit a business.
Also from my state's handbook:
"Confused roadway users - People who are confused may cause an unsafe situation. People who may be confused include:
• Tourists or others, who do not seem to know where they are going
• Drivers who slow down for what seems like no reason
• Drivers looking for street signs or house numbers"
Of course the person who sees a yellow light and stops to allow someone to exit a parking area isn't confused in reality, they know why they slowed down or stopped but the cars around them don't know that.
The more unpredictable you are the more unsafe of a driver you are. Don't get all road rage on someone who does that but don't confuse that action as being safe either. Neither are really correct here.