You're confusing specialized refrigeration maintenance (which, at least in the US requires EPA certification) with general end user operations tasks. It is the store manager's job to know how to operate, optimize, and take basic care of their equipment. Just like it's an excavator operator's job to know how to check the fluids and grease the machine. They don't need to rebuild the engine or do specialized maintenance like lineboring, but they are responsible for basic care of the machinery.