Contrary to what you would think as a consumer, grocery chains almost always want you to buy their brand (the less expensive one) because the margins on store-brand items are much better. That's why the stores bother selling them in the first place (and why margin-hungry companies like CVS will have every popular placed side-by-side with a store brand)

Name brands pay for premium shelf/fridge space.

And that is probably the reason for the milk up front. The name-brand milk producer is paying big bucks for space in that cooler near the checkout, enough to offset the supermarket's probable loss in sales of their own store-brand milk in its normal location in back.