In what would seem to have been a no-brainer, a three-judge panel of the Eighth U.S. Court of Appeals in St. Louis ruled last year that prosecuting attorneys should not receive blanket immunity from lawsuits and monetary damages when they use false evidence to obtain a conviction in a criminal case.

The ruling applies only when a prosecutor has obtained, manufactured, coerced and fabricated evidence before filing charges and then used the evidence at a trial.

In other words, if you discover that a prosecutor cooked up evidence and used it to charge you with a crime, and then presented the evidence at trial, you now can sue him if you discover what he did.

This ruling hardly seems controversial. But it bucks hundreds of years of Anglo-American jurisprudence. Prosecutors have enjoyed near complete protection from civil liability for misconduct during criminal prosecutions. Prosecutors from Pottawattamie County, Iowa, appealed the Eighth Circuit’s decision.

Last week the U.S. Supreme Court heard arguments in the case. A decision is expected this spring. The appeals court ruling should be upheld. Prosecutors shouldn’t have immunity if they frame suspects.

The case stems from the 1977 shooting death of John Schweer, a retired police captain who was working as security guard at a Council Bluffs, Iowa, car dealership. Two teenagers, Curtis McGhee and Terry Harrington, were convicted of murder in 1978. In 2003, they were released after it was disclosed that prosecutors had fabricated the testimony of a lead witness and failed to disclose that there had been another suspect.

The two men then brought suit, alleging that prosecutors had framed them for the crime by fabricating evidence during the investigation and then presenting that evidence at trial.

Until now, a prosecutor would be entitled to complete immunity from civil liability, even when he or she knowingly presents perjured testimony at a trial that leads to a wrongful conviction. The rule is designed to advance some fundamental principles of public safety: Prosecutors should be permitted to pursue criminals with “courage and independence,” unhindered by worries about personal liability.

Under existing law, police who fabricate evidence receive no immunity from civil lawsuits. Nor does a prosecutor who fabricates evidence during the investigation but does not participate in the case once charges are brought.

But the prosecutor who phonies up the proof during the investigation and prosecutes the case with that false evidence is home free — completely protected from civil liability.

As Justice Anthony Kennedy put it during last week’s oral arguments, “So the more deeply you’re involved in the wrong, the more likely you are to be immune? That’s a strange proposition.”

Even Deputy Solicitor General Neal Katyal, arguing a friend of the court brief on the side of the prosecutors, acknowledged, “It does seem a little odd.”

Strange and odd, indeed.

Prosecutors argue that carving out even a narrow exception could lead to a flood of contrived cases. Considerable resources would have to be diverted to defend these cases. Prosecutors, meanwhile, would pursue cases defensively.

That concern is understandable, but it’s not convincing.

Former U.S. Solicitor General Paul Clement represents the two Iowa men. He argued to justices that, in this case, the exception is sought for “pre-trial misconduct (that) both shocks the conscience and was designed to subvert all the procedural protections afforded suspects and defendants.”

This level of immunity would expose dirtiest imaginable prosecutors to liability. A legal system that insists on protecting that level of misconduct risks squandering of public respect.