Anyone with warehouse stock from multiple manufacturers has to deal with suppliers too. RA is just cutting out the middleman, which can make sense when merchants don't need to shuffle parts around from one location to another, or buyers don't need the support of a local brick 'n mortar, nor an immediate local purchase location. As a result, they have removed their primary chance to do quality control while in possession of the product, and increased your burden to return items vs buying local, but you have to love their website, selection, and low prices.
The thing that gets me about their A/B/C warehouses is that sometimes funny things happen. You can have parts from warehouse A for example, and be shopping for more parts from warehouse A so you don't end up paying for shipping from two different locations, but then when you add the new parts from warehouse A to the order, somehow some of the prior parts that were shipping from warehouse A, change to shipping from a different warehouse so you're still stuck with two separate shipping charges. It can be very frustrating when that happens.