It is in the best interest of big box retailers to carry the latest licences, for MANY reasons. SALES also draw customers, those who have been holding off on 7 will now invest. The software licences seem about the same, the hardware they come preinstalled on gets pretty cheap though.
A shop that doesn't carry mainly new products and software does not look like they are on top of technology. Stock movement is also critical, whereas in a boutique style shop, it is less important as the solution provided.

As for Premium price, the higher end machines are still the higher end/premium machines, around here they still mainly boast Win7, but you'd never see anything more than halfway decent in a big box retailer, FutureShop/Best Buy etc..