Costco is one of the few stores that sells tires that actually honors tread wear warranties. They also include road-hazard insurance rather than charge extra for it.
Like any business they want to minimize the cost of returns. They get credit back from the tire manufacturer for tread wear returns but few people take advantage of this even though they are entitled to. Road hazard replacements aren't covered by the tire manufacturer, so they want to minimize them.
If everyone was good about keeping tires properly inflated then there would be little upside in inflating with nitrogen. They aren't so there is.
Remember, even though some tire stores may charge $5 per tire for nitrogen, the actual cost to generate that nitrogen is about 1/100th of that.