As an Australian, I don't think I could dream up a stranger supermarket item to have such a theft problem. We of course do have the whole kangaroo carcasses kept in the freezers with the grid mesh doors securing them.
I think the issue is that the Spam is an easily fenced item in Hawaii, thus it is a target for theft. Since it's a hassle for both staff and customers to conduct a sale if the Spam is under lock and key, it must be a serious problem for them to go to that kind of effort to secure it. If Spam is a black market commodity, they may have to become serious about securing it.
The shelf locks may also be part of a generic system, perhaps they're used for locking up higher value items such as razors and pharmaceutical products, so it's reasonable that they would use high security locks for this application.