Yes, they have egress locks activated by magnet that locks the wheels, or they have another physical lock system, at each pedestrian exit to the car-park of my local supermarket -- that's one reason they are really inconvenient for pedestrians accessing the site, they want to have trolley controls.

The exits are narrow with soft soil and low hedges bordering the paths. You can lift a trolley over, but you need some muscle, especially if your trolley is full of shopping.

Push the cart quickly enough and you can defeat the magnetic locking mechanism. Just push it really fast past the boundary where it would lock and keep going until you’re out of range.

Dunno if that still works, but I enjoyed defeating the locking mechanism at Safeway when I was a kid even though I didn’t even want the shopping cart.

Yep, that's the real reason. Introduced first in France and not by Aldi.

And yet... haven't you noticed that supermarkets in posher areas are less likely to have the coin slot? (Asda and Tesco are two whose policy varies by location).

And in supermarkets with large car parks there are return stations all over the car park. Which means you still need to pay trolley herders...

I am not in America, I can't therefore share your observation. But it is not news that crime of all sorts is lower in "posher areas". Of course it is. What's the point you want to go for?

I think the posh bit is slightly incidental, I think it's shops that people [can] walk to.

Never played Sim City? Zonal planning is not incidental.