Agree, it's complete nonsense. I used to work on self-checkouts in Morrisons, and I was 1:4. I could have handled 1:6 quite easily (which seems to be the current setup in Morrisons). But the ratio that's being seen in Tesco and Asda (as the two worst in my experience) is dire. Although, in my experience, the staff could also be working more efficiently and proactively. It's very common to see staff on the self-checkouts having personal conversations with colleagues and random members of the public. I was always either at the head of the queue, greeting customers and directing them to the next available checkout. Or helping the customers. Or thanking the customers as they left and preparing the checkout for the next customer if needed. Even with only four tills, I could always keep myself busy. So I fail to see how someone looking after 16 has time for a chat.
If done properly, you can get 2 self scan facilities in the space taken by a regular checkout. This means that a 1:2 ratio should be "break even" for the supermarkets. Sadly, tesco seem to have a habit of replacing x number of tills with the same number of self scanning machines, and then stretching their workforce even further to cover 14-20 in one go.