But given how many issues you get with every other aspect of the self checkout process (items failing to scan properly, machines being broken, "unexpected item in bagging area", etc) that seems to be a very generous assumption.
And when someone has had to wait in a queue to use on of the working self checkout machines, has to go through scanning all of their own shopping (which is stores have managed to get customers to do for free so that they can make more money), has to wait for someone to come over to "assist" them when the machine complains about something, and is then (directly or indirectly) accused of trying to steal from the store because of a faulty self-checkout machine, it's not a huge surprise when they get pissed off about that.