The beer battle for shelf space is under regulatory scrutiny as the Department of Justice reviews Anheuser-Busch InBev's distributor acquisitions over fears it may wield too much power.

Even though Anheuser-Busch InBev (NYSE:BUD) sold almost half of all the beer consumed in the U.S. last year, it feels so threatened by flagging sales that it has attempted to stop craft brewers from getting their beer onto store shelves.

According to Reuters, the Department of Justice is investigating Anheuser-Busch over its acquisition of five distributors in three states over the past few months, which craft brewers say has made it extraordinarily difficult to distribute and sell their beer.

Coming up just as Anheuser-Busch is attempting to acquire rival SABMiller (NASDAQOTH: SBMRY) in a mega $108 billion merger, the probe could derail, or at least delay, the deal's completion. There are already concerns about consolidating so much market power into the hands of the two leading brewers, and this investigation will surely heap fuel onto the fire of what competitors might expect with an even bigger, more powerful industry leader.

Crying in its beer
Craft beer has been a thorn in the side of Anheuser-Busch. Despite the mass brewer's size and heft, only craft beers have been enjoying growth.

According to the Brewers Association, craft beer continues its torrid rate of growth with U.S. production volumes rising 16% across the first half of 2015, hitting 12.2 million barrels for an 11% share of the market. There are now more than 3,700 craft breweries operating in the U.S., up 22% from the year-ago period, while 1,750 more are in the planning stage.

Mass-brewed beer can only look on longingly. Between 2009 and 2013, U.S. beer volumes fell 6%, and dropped another 0.6% last year. Volumes have fallen for Anheuser-Busch as well, which despite higher sales per unit volume in the U.S., still suffers from declining volume sales. In this year's second quarter, it reported beer volumes fell 2.1% as it no longer benefited from the lift World Cup soccer provided.

Considering the U.S. represents Anheuser-Busch's biggest market -- it has a 47% share -- accounting for 24% of net volumes and 30% of net revenues in 2014, the continued slide is worrisome.

Crafting a comeback plan
The company has been trying to make up for the shortfall by buying up craft brewers. In the past month alone, Anheuser-Busch has purchased L.A.'s Golden Road Brewing and Mill Street Brewery in Toronto, while it acquired Elysian Brewing in January. Last year it bought both Blue Point Brewing and 10 Barrel Brewing, and the year before that it snagged Goose Island. It previously launched Shock Top in 2006 as a response to MillerCoors' Blue Moon.

The DOJ probe into its acquisition of distributors suggests the company is trying to game the system in favor of its own craft beer brands at the expense of the competition.

When it bought two Colorado-based distributors this past August, Anheuser-Busch was quoted as saying, "American Eagle currently sells approximately 20 non-AB brands. We are seeking to keep a handful of local brands, pending supplier approval". That meant a bunch were going to get the heave-ho.

In addition to their small size, craft brewers also have to contend with a distribution system that favors large brewers like Anheuser-Busch. 

A labyrinthine regulatory system
The problem for craft brewers is the antiquated distribution system they're required to operate under that favors mass brewers like Anheuser-Busch and Miller.

Following Prohibition, the government imposed a three-tiered system on the industry requiring brewers to sell their beer to a middle man, the distributors and wholesalers, which in turn sell to retailers, whether they're package good stores, bars, or grocery stores.

The idea was to prevent the brewers from owning the retail operations and limiting consumer choice while hiking prices. But regulators have a poor ability to foresee the future, and now craft brewers are placed at a competitive disadvantage as the mass brewers buy up the distributors and crowd out the competition.

Four of those mass brewers -- Anheuser-Busch, Heineken, SABMiller, and Carlsberg -- make up 70% of global beer sales. The investigation likely will raise some uncomfortable questions for Anheuser-Busch InBev as it attempts to further solidify its industry-leading position by snapping up SABMiller. But it also just might lead to this outdated model of distribution being dismantled, which could upend the beer giant's attempt to squash competition before it even reaches store shelves.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.