Medicare denial rates higher than private insurer's

One argument often cited in support of single payer health care or the need for a public option is the idea that health insurance companies have an incentive to reject legitimate claims because they are motivated by profit. I find that argument similar to saying that because WalMart is motivated by profit, that it has an incentive to not honor its return policy, or that Apple has an incentive to not honor IPOD warranties, or that McDonalds has an incentive to serve its customers spoiled food because it might save them a few bucks. The reason that this is not necessarily true is because profit motivated companies have an incentive to protect their reputations, and health care insurance companies are for the most part, no different.

And if the proponents of government health care are correct, then why, according to the AMA's National Health Insurer Report Card, does Medicare apparently reject claims at a higher rate than private insurers? And why did Aetna, who rejected 6.5% of its claims in 2008, in 2009 reject only 1.81% of claims in 2009? Did Aetna all of a sudden become less concerned with profits?