It wouldn't have happened in my local WHSmith as they have no staff!
Myself and another lady were waiting to pay the other day. In the end I went to find a member of staff. When she served me, she said that "people usually use the self serve". I responded that "two of us didn't want to use the self serve" She then explained that there were only two members of staff in the store and that they had other jobs to do. (Basically saying that she didn't want to serve me as I was being an inconvenience).
When I was younger I worked in Primark on a Saturday for about 7 years, so have some idea as to what is involved in retail and how busy it can be. I would have been expected to stop what I was doing to help on the tills if there was a queue, and would do so without being asked. We were also expected to keep an eye out for shoplifters and report to a manager or security guard if we noticed anything.
Security wise anyone could have walked out of that smiths with anything as there was no security guard and the only staff member was at the back of the store.
I have got to the doors, or once even out in the street, and then realised I had something in my hand, and walked back in to pay for it. When I have said what had happened the staff weren't phased by it at all. Did get told I was honest for going back in. When your mind is already thinking about where else you need to go and what else you need to do, it can be easy to forget what you are actually supposed to be doing there and then.
Sounds like it was a bit of an over-reaction, but maybe you weren't the first that day.