Most 2-3y old kids can walk and when my babies where too small to walk I would just put my groceries in the bottom part of the stroller and in an hiking backpack instead of a cart.
I get that you don't want to make accommodations for people but you might want to think about what leads you all to get so bent out of shape on this one. At the end of the day your argument boils down to "your spouse just needs to work harder so I'm not inconvenienced by the cart she hung off the curb next to the space I wanted." Society is full of people with differing abilities in differing mental spaces and not everyone owns equipment like that and is willing to spend 10 minutes getting it out and setting it up in a busy parking lot while also wrangling their willful, very curious child.
There are absolutely lazy people, but it's not always the case.
People having newborns typically use baby carriers and strollers and when using the later use the storage space of the stroller to carry a significant amount of groceries. Nobody with a sane mind woud grab 2 newborns in the same arm and throw them in an unsafe shopping cart.
Every single person that doesn't return their cart does so out of laziness. Besides just being an asshole, the cart will take a potential parking spot that someone else later needs to move to free up, and worst of all the wind could blow the cart into someone elses car.
Nobody is gonna kidnap her kids as she walks the cart back in less than a minute. It is simply her being a lazy asshole.
I can see that this is a very personal issue for you, so I'll just say this: People are complicated, and I would encourage you to have more grace for them. If it bothers you that much to see a cart left by a mom struggling with kids, you might consider offering to return it on her behalf.