I generally try to use "about the right amount of force" when closing doors and see no need to slam them...
...that being said, I am often surprised that I leave a door slightly ajar...usually the dome light doesn't go off.
With cars, they are a bit air-tight these days so as you are shutting the door- there's a brief bit of air pressure fighting against you.
So, I'm sure there's a bunch of folks that just don't get it...on the other hand, I'm sure there's some folks who slam because they have a valid worry that the door won't close.