Not valid. The expectation of what to do with your shopping cart varies regionally. Some places actually send someone out WITH you to put your groceries into your car; is accepting such help unethical? Why is it unethical to accept the help of someone hired to round up your grocery carts?
In the last couple of decades, the trend has been more and more places scattered throughout the parking garage to return the carts to. When I see those, which is almost all the time now where I live, I take the hint. But they still round carts up FROM those places to bring into the store. So is using those only partly ethical?
A better test would be people who leave groceries they’ve changed their mind about on the shelves in the wrong place. If the groceries are spoilable, you’re positively evil.
People that do that really grind my beans. I hate lazy people like that and worse yet people like my wife (who works in a supermarket) have to waste their time picking up after lazy slobs who do this.