There are no laws on oligopolies
The threshold of 5 firms holding 80% of the market - which IIRC is the usual definition of oligopolies - is not part of the Sherman Antitrust Acts. Only monopolies, which I think kicks in where a single firm holds 80% market share, is the only time the government steps in - and there's been only a handful of these cases - the breaking up of Standard Oil into the Seven Sisters is the only case of substance.
I'd agree that government has a place in preventing excessive market concentration by a small number of firms (both Monopoly & Oligopoly), but using that to justify the intrusiveness of government in business is a bit disingenious. Government not only does not regulate the market share of these companies, but nine times out of ten, government policy has the effect of entrenching these big interests even further.
