This sounds to me very much like a typical large-corporation lawyer/self-preservation issue.
Sears is most likely going by the recommended tire PSI in their computer. I have bought tires from them in the past and the clerks have nearly blown a gasket when I would request a 235/75R15 for a vehicle that had 215/75R15s listed in their system as the correct OEM tire.
Most likely this is a way to avoid liability in case of a claim of a burst tire or handling issue where the customer comes back and says "They overfilled my tires, causing a failure, and therefore I will sue them."
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