This post that was quoted has been deleted.
Quite simple to understand really. All mercadona stores have rigid plastic shields to seperate the cashier from the customer. If you push the trolley front in it is impossible to unload and also puts you nearer the customer in front. Turning the trolley allows you to easily unload the trolley onto the the belt. The customer behind should, and in my experience always will, be the required distance from you as marked on the floor. Why do some people make life so difficult and whinge about every little thing. It's just a little ask and is to protect their staff and you. Maybe the posters should lighten up, far worse things to worry about.