The U.S. Department of Justice is preparing to go to court toblock two giant health insurance mergers, adevelopment that is surprising, but not shocking.

Thus far, the four companies involved in the two proposedmergers Anthem-Cigna and Aetna-Humana have kept their public response to antitrust concerns relativelyvague, arguing in general terms that their increased size will notsignificantly reduce competition in the insurance market but willallow them to offer better service to members.

By merging, they have argued, they will become more efficientand stronger in price negotiations with hospitals andpharmaceutical companies, which will lead to lower costs for theirmembers.

Aetna will have to convince a judgethat its acquisition of Humana, one of the leading providers ofMedicare Advantage plans, the private sector alternative totraditional Medicare, will not significantly harm competition inthat market.

Some expert observers suggest, however, that the companies mightbe able to point to the continued existence of traditional Medicareto counter the evidence that they will indeed dominate the MedicareAdvantage market.

Considering that two-thirds of seniors still opt for thegovernment-run Medicare plan, a highly consolidated privateMedicare market might not be viewed through the same lens as mostantitrust battles.

Mark Rust, an antitrust attorney, explained to CNBC that whilethe proposed Aetna-Humana company would account for a quarter ofthe Medicare Advantage market, it would only occupy 5 percentof the overall Medicare market. Five percent is hardly amonopoly.

That type of argument is not going to be available to Anthem andCigna, which are mostly vying for the non-senior private insurancemarket. While millions of non-seniors are covered by public healthprograms, notably Medicaid and the Veterans Administration, both ofthose programs are only open to certain segments of the population,and therefore do not truly compete with the private sectorinsurers.