I don't think that's a fair comment. I think yes there are people who are in a rush when they don't have to be, they are the same people who can tailgate you all the way only to end up at the same light as you lol.
Sometimes, and I do mean sometimes, you have cars coming in both directions and/or you're on a main pathway and then someone decides to back into a spot. It's a situation that may not take much time for you (although I know we're all just exaggerating that really it only takes 10 seconds longer because to be fair I don't think many of us have actually timed ourselves on the regular to see how long it takes us to park the point you're trying to say and others is subjectively it doesn't take all that much longer) but it can cause a ripple congestion factor.
Backing in isn't really an issue, but I think situationally speaking there are times where it's best not to and if someone is deciding to not care about their surroundings and do it anyways well that is often the situation you'll see people complaining about.
Even though most people said in Costco it doesn't make sense to do it I'm going to use their parking lot to give a better visual.
Here's the closest Costco that we go to. I marked the main pathway into the parking lot that people use to get to the other side of the parking lot. Both directions I marked are problematic for someone to decide to back into a spot. It would cause congestion problems and it would be an issue there not about someone not being able to take 10 seconds but because the condition of the area it's not conducive to do so. And again I'm only using Costco's parking lot, despite that people said they wouldn't back into spots there (I agree you need access to your trunk more) and Costco's parking spaces are wider anyhow but it's just to illustrate what I was talking about
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The same can be said for our mall, when it's really busy and people are just trying to find a spot any spot that backing in has a tendency to create problems.