The only "cart returner" I saw against it basically just claimed that the people in their town/state/country were too incompetent to operate shopping carts (even if that's not what they explicitly said) so idk if i really trust them or want to use that as a measure.
Making work for others to save yourself some trivial amount of work absolutely says something about your character
That only applies if its someone in your social circle. You can follow up with them and ask why, learn about their struggles or stance on it.
With strangers you have none of that, just little glimpses into their life for a few seconds as they cross yours. When we are out in public it is very important to make quick judgments for safety, but this often is confused with moral judgment.
There are very few people in each persons social circle that they know well enough to judge morally. Strangers aren't close enough by a long shot.
With all this considered, I have to conclude its best to always give strangers the benefit of the doubt when personal safety isnt involved.
I'm concerned this post is showing people are coming to the opposite conclusion, that we now have this great new way to judge strangers we shouldnt be judging to begin with.
For complete strangers who you never see again, it doesn't matter if you judge them or not, you'll never see them again.
It's useful on aggregate to tell the general attitude of an area to set expectations for interacting with strangers.
But mostly what people are talking about is when you're getting to know someone. If you find out an acquaintance or romantic prospect does/doesn't then it's one (of many) indicators you can gather to build a model of them.
Remember, this isn't deep. This is intentionally super simple. There is no "struggle" involved that wouldn't be immediately apparent. There is very little room for nuance because there is very little to be nuanced about.
This is "given the chance, will this person spend a trivial amount of effort to make someone's life easier, if there is no personal gain?"
First, what absolute savage would take a new acquaintance or romantic interest grocery shopping and make a show out of leaving the cart? Thats absolutely a red flag.
Second, you are right that with strangers it won't matter if you judge them, but thats only true from their perspective. The way you think and carry on affects how you feel, its not a good idea to waste energy judging people in that way. It affects how you perceive and interact with the world essentially.