In U.S. v. Dawson, No. 04-2557, federal agents and an informant agreed that in exchange for cooperation (which included testimony), the informant would take 20% of any forfeitted money. Since most cases plead out, the government informant would usually obtain his cut off the action without having to testify. But in Dawson, the case went to trial. And thus, the informant had to testify in order to obtain his fee. Paying a witness to testify is a felony. Posner agreed with this:
And the defendants are right to point out that paying witnesses (other than experts) for their testimony (beyond the tiny fees permitted, in the case of federal trials, by the Judicial Code, 28 U.S.C. § 1821) is forbidden. 18 U.S.C. § 201(c)(2). Even an expert witness, and a fortiori an occurrence witness, may not be paid more if the party for whom he is testifying wins the case.
Should government officials be able to commit a felony by putting the informant on the stand? Look how Posner characterizes the federal law:
Yet whether violation even of that rule requires exclusion of the testimony from being used against a defendant is a separate question.
Title 18 U.S.C. § 201(c)(2) is not just a rule: it's a federal criminal law, carrying with its violation felony penalties. Shouldn't trial courts, under their supervisory powers, prevent felonies from taking place in their courtrooms? Posner continues:
Exclusion confers windfalls on the guilty and therefore, at least as a device for enforcing nonconstitutional rules, is disfavored.
But if government agents are violating a federal criminal law, aren't they guilty? Doesn't allowing them to admit illegally-obtained evidence confer a windfall upon those criminals? Doesn't it cheapen the judicial process to require judges to sit on their hands while felonies are committed in their courtrooms? Posner ducks those questions, and then dresses up his unwillingness to enforce the law as a matter of separation of powers:
Judges are in no position to evaluate the government’s need to offer monetary or other inducements to the criminals whom it hopes to enlist in the "war against drugs."
No, but judges are in a position to determine whether the government, by paying a witness for his testimony, is violating the law. Judges are in a position to ensure to preempt felonious conduct. This is yet another application of this maxim: The government may break the laws to enforce them.