Because its a $10 battery and people don't often take advantage of the deal, US Cellular is able to easily eat the degradation cost. But on a $40k battery pack,

Because its a $10 battery and people don't often take advantage of the deal, US Cellular is able to easily eat the degradation cost. But on a $40k battery pack, where swapping happens potentially far more often, that cost is too significant to ignore. Hence they're charging a reasonable amount for it.