[Baz]

If you want to break this down analytically - the person to blame is the person who was next in line. It's THEIR responsibility - not the cashier - to govern and/or manage who is served next.

Ever sit in a line of cars at a light - the lights changes - and it takes the cars in front of you twice-three times the necessary amount of time to clear the light so you can get through?

Or - when a traffic lane merges.......how that is managed by those in the main lane - how they let merging traffic in.

Same exact deal in a store.

Shouldn't come down to fisticuffs - you have to use your body (or vehicle) to send the right message. Eye contact and hand gestures are also very helpful.

It's all about awareness.

Many are pretty clueless, and thus civilisation suffers for it.