[scruple]

And inevitably there are always at least 2 employees standing around the self-checkout helping customers that are having problems with the machines.

[Jtsummers]

2 employees assisting with 6-12 self-checkout stations is still a net-win for the employer if the objective is cutting "overhead" of personnel. It's only a problem if the self-checkout system is flawed in a way that requires frequent and lengthy intervention by the employees, versus the occasional ID check for alcohol purchases and item check for something not scanning correctly.

[smichel17]

The "problems with the machines" are almost exclusively anti-theft "features".