To encourage the purchase of health insurance, the Affordable Care Act added a number of deductions, exemptions, and penalties to the federal tax code. As might be expected from a 2,700-page law, these new tax laws have the potential to interact in unforeseen and counterintuitive ways. As first discovered by Michael Cannon and Jonathan Adler, one of the new tax provisions, when combined with state decisionmaking and Interal Revenue Service rulemaking, has given Obamacare yet another legal problem.

Here’s the deal: The legislation’s §1311 provides a generous tax credit for anyone who buys insurance from an insurance exchange “established by the State.” The provision was supposed to be an incentive for states to create their own exchanges, but only 16 states have opted to do so. In the other states, the federal government established its own exchange, as another section of the ACA specifies. But where §1311 only explicitly authorized a tax credit for people who buy insurance from a state exchange, the IRS issued a rule interpreting §1311 as also applying to purchases from federal exchanges.

This creative interpretation most obviously hurts employers, who are fined for every employee who receives such a tax credit/subsidy to buy an exchange plan when their employer fails to comply with the mandate to provide health insurance. But it also hurts some individuals, such as David Klemencic, a lead plaintiff in one of the lawsuits challenging the IRS’s tax-credit rule. Klemencic lives in a state, West Virginia, that never established an exchange, and for various reasons he doesn’t want to buy any of the insurance options available to him. Because buying insurance would cost him more than 8% of his income, he should be immune from Obamacare’s tax on the decision not to buy insurance. After the IRS expanded §1311 to subsidize people in states with federal exchanges, however, Klemencic could’ve bought health insurance for an amount low enough to again subject him to the tax for not buying insurance.

Klemencic and his fellow plaintiffs argue that they face these costs only because the IRS exceeded the scope of its powers by extending a tax credit not authorized by Congress. The district court rejected that argument, ruling that, under the highly deferential test courts apply to actions by administrative agencies, the IRS only had to show that its interpretation of §1311 was reasonable—which the court was satisfied it had.

Cato and the Pacific Research Institute have now filed an amicus brief supporting the plaintiffs on their appeal to the U.S. Court of Appeals for the D.C. Circuit. While it is manifestly the province of the judiciary to say “what the law is,” where the law’s text leaves no question as to its meaning—as is the case here with the phrase “established by the State”—it is neither right nor proper for a court to replace the laws passed by Congress with those of its own invention or the invention of civil servants. If Congress wants to extend the tax credit beyond the terms of the Affordable Care Act, it can do so by passing new legislation. The only reason for executive-branch officials not to go back to Congress for clarification, and instead legislate by fiat, is to bypass the democratic process, thereby undermining constitutional separation of powers.

This case ultimately isn’t about money, the wisdom of individual health care decisionmaking, or even political opposition to Obamacare. It’s about who gets to create the laws we live by: the democratically elected members of Congress or the bureaucrats charged with no more than executing the laws that Congress passes and the president signs.

Halbig v. Sebelius will be heard by the D.C. Circuit on March 25 (the same day that the Supreme Court hears the Hobby Lobby contraceptive-mandate cases).