Anyway, the fact that the two companies have an 80% market share is part of the economic calculations that go into anti-trust investigations, but it is not the only thing.
One of the most important concepts in anti-trust is the contestability of markets: how easy is it for a competitor to come along and compete against you? And in this the beer market is pretty contestable as shown by the rapid rise of craft breweries. In fact, the very success of craft brewing over the last decade would be exhibit number one in an anti-trust hearing - this success shows that beer is an easy market to get into and thus there is intense competition. Now most of this competition is local and the big breweries have vast economies of scale on the national level, but their ability to rise prices is severely limited by fierce competition in local markets from craft brewing.
Or course this can be seen another way, because another concept in anti-trust that factors in is how we define the market. Are macro-brews and micro-brews really in the same market and thus micros are in fact real competition for macros? (And for that matter are all alcoholic beverages actually part of a single market so that the real share of these companies is quite small?) I have wondered in this blog many times about how high is the cross-price elasticity of micro and macro beer for this very reason, for it is with this figure that we decide how close the markets are. In other words when the price of Deschutes beer rises, how big a bump in the demand for Bud is there?
I have to admit that my sense is that the AB InBev and MillerCoors could make a pretty spirited defense of their case in an anti-trust hearing. They could show how in local markets beer sales are very competitive and that the national economies of scale they enjoy are not easily leveraged in local markets to keep prices artificially high. To me, the decision to raise prices just seems like simple profit maximization - and good news for local craft brewers as it appears there will be more market share to be had.