If they hired an extra 20 managers across 1000 stores to deal with twiddling fridge knobs over the phone, we'd be complaining about that instead. And someone would still call up managers and social engineer them to setting the knobs wrong.
And social engineering 1000 managers is FAR more work than scaling a software hack across 1000 machines, and also far less likely to succeed. At least one of those managers is going to get suspicious and call corporate first. Moreover, it will be pretty hard to get an on-site manager looking right at the full freezers to set them to defrost....