It's more equipment to maintain. One maintenance person can only do so much. The more equipment there is to maintain, the more personnel needed to do the work, therefore they have to hire (or contract for) additional maintenance personnel. Those maintenance personnel are frequently paid far more than a cashier, and not much technical education is required to do the job. It's not uncommon to find college tech students doing on call maintenance or tech support service, as their hours allow them more of a flexible schedule.

So, where are the savings?

The Wal Mart in Norman, OK took out two regular registers, and put in 6 self serve scanning stations. They cut back to one express checkout lane. They still have 2 cashiers monitoring those self serve aisles, but they can serve many more customers. Fewer lines, more sales, happier customers.