It's interesting to hear what you say, however I have to come to the aid of the checkout staff having been one myself during a part time job.
All the major supermarkets employ companies that check up on them and the way customers are treated by the staff. They are called Mystery Shoppers. This person may turn up at any time, wander around the store, buy a couple of things and go through the checkout in the normal manner. They do not have to announce themselves to anyone.
The checkout person however has many specific guidelines to work too. In my case, 19 items a minute had to be passed through the till, the customer had to be greeted in a pleasant manner and I was obliged to ask the customer whether they were able to pack their own bags or should I do it. The bill also had to be correct and the correct change given.
If any of the aforementioned criteria was missed when the mystery shopper came through the till, a report would be generated by their company. If two reports were received in any quarter of a year, all the staff in the store lost 1% of the annual bonus. The bonus was a maximum of 3% of their annual salary. To lose one third of it was very easy.
As many of you probably know, dealing with the general public is not easy. Some want to stop and chat which holds upthe queue, others want to get their shopping and get out of the store. At some point staff are also entitled to an off day and maybe not treat the customer as he/she expects.
This post is not intended to make excuses but to point out that the staff can be under pressure too. Treat them as you like to be treated is my motto.