TFA explains this. Beam, Inc. strikes a deal with the distributors saying "if you want Maker's you also have to buy Skinnygirl". Once again, if they wanted to dilute it for specific markets, why did they do it for all the markets?
I did think of an alternative explanation, though I think this is less likely. Perhaps Maker's sells well at their current price point because there it's a good value. However, if you raise it by, say, $10/bottle, it will now compete with more premium bourbons where it will not hold up nearly as well. Then, lowering the price on it after the shortage is over will even further cheapen the brand. So perhaps Beam, Inc. was faced with a really tough choice of whether to market Maker's as even more premium and lose, or lower the quality and hope that the customers won't mind. However, I think TFA explains it better, and Occam would probably agree.
I agree with you about price vs. demand; I think if you read between the lines what Beam already said, they're more or less acknowledging this (in that you can always solve a demand problem by increasing the price, and they're choosing not to do that).
If they allow the price to rise, bar purchasers move to alternatives. (Whether independently or in conjunction with a shift at the distributor level.)
Currently, Beam can and does (per the article) use Makers Mark as leverage to sell their other products. I would imagine including before the point of the bar's purchase decision. Liquor distribution is big business in the U.S.; in many places, it is an oligopoly if not monopoly. (Illinois, for example, currently has issues with this. And I realize that this circumstance might argue for any potential change taking place at the distributor level rather than the level of the individual bar, depending upon whether and how much diversity the distributors offer.)
If you want Makers Mark, it comes as a package deal along with X quantity of this other stuff. Being that the other stuff is targeted at less discerning market segments (at the least, based upon price), a pretty good argument can be made that "whatever you buy for these segments, you'll be able to move it" (especially with a little effective marketing).
One of the first words that popped into my own mind was "bundling". Whether the article has the situation right, this is how I understood what it is saying. (With "bundling" being my word and analogy, whether or not it would be the proper term and analogy.)
I'll reiterate though, this is not an uncommon position for a bourbon brand to be in. Bulleit is a Diageo brand; one can only imagine the crap Diageo might want to bundle. Even Sazerac has a bunch of crappy mixers and vodkas in their portfolio.
I'm also not entirely sure how your logic works here. How does reducing the proof on Makers help Skinnygirl Pre-mixed Processed Cocktail Beverage Product in any way? Those products are presumably already marketed to capture profits for markets that don't care about quality.
I buy Beam's explanation of why they did this. They pivoted Makers as a global brand, and in foreign markets (like Australia) were already marketing the product at a lower proof. They were unexpectedly successful, and one sane response to shortage was to unify the product line around the foreign proof.
Obviously, Makers has always controlled the final proof of its product. Bourbons sold cask-strength are sold that way in part because it allows the distiller's customers to buy less water and more booze. The final proof Makers tried to choose wasn't an unusual one for bourbon, either.