I prefer dark, heavy beers, but usually only in the winter.

Yes, the last of this year's Sierra Nevada Celebration disappeared from the shelves at the end of January, but a few six-packs of Pyramid Snow Cap are hanging on...

What do you mean "locked up"?

In the U.S. you certainly see places like airport bars, franchise restaurants, and even neighborhood restaurants with a limited number of taps and cooler space where the choices are Bud and Bud Light (Oh, and maybe Killians for the "dark beer!!). Beer distributors still market pretty aggressively, but I don't know if they're allowed by to negotiate "exclusive" contracts the way they used to (I'll lock in a great discount on Angst Lager, so long as it's the only beer you serve).

In Mexico (and some trips to Puerto Vallarta are my only recent experience) I don't know what the trade laws are like, but it sure looks like they allow exclusives or at least serious promotional deals. I've had that "Dos Equis obscura" conversation 30 or 40 times at various beach bars where Sol or Corona banners and sun umbrellas are flying and where the only beers available are what's on the umbrellas.