It's a bit more complicated than that. Brewing used to be a vertical industry with the brewers owning most of the outlets. Many still do but there are now more retail chains (known as pubcos) who usually enter into an agreement with a particular brewer to sell only their products.

it's not such a restriction on choice when there's a rival pubco a few yards down the road but a couple of decades ago you would have found whole towns monopolised by a single brewery.