WASHINGTON — The Affordable Care Act is the biggest new health care program in decades, but the Obama administration has ruled that neither the federal insurance exchange nor the federal subsidies paid to insurance companies on behalf of low-income people are “federal health care programs.”

The surprise decision, disclosed last week, exempts subsidized health insurance from a law that bans rebates, kickbacks, bribes and certain other financial arrangements in federal health programs, stripping law enforcement of a powerful tool used to fight fraud in other health care programs, like Medicare.