It's so rare here I can't think of another time I've seen it. I've always wondered why: comfortable people tend to be less grumpy and the typical cashier job involves "not leaving a very small space for a long period of time". This small space is about the size of ... a chair.
I think the perception of people standing in line is more likely to make them feel that the cashier is working slower because they're sitting[1]. Aldi's lightning quick cashiers would prove that wrong, but perception tends to be all that matters in these scenarios.
[1] People routinely over-estimate their waiting times in situations like this. This became apparent to me when talking to the helpdesk manager at my old company. He would get an e-mail from time to time from a person complaining that they were on hold for "20 minutes". The call stats for the day indicated a maximum hold time of 5 minutes.
The difference is probably the combination of high footfall and continuous restocking in food shops, which results in role specialisation and hence a more static workforce where the role is static.