"It's called flipping and it almost ought to be illegal."

That was the President of the United States this morning, according to this CNN article:

Trump's latest attempt came in a friendly taped interview with Fox News, which was conducted on Wednesday but aired a day later. Trump sought to put distance between himself and his former lawyer Michael Cohen, who admitted to campaign finance crimes in federal court on Tuesday and implicated the President by saying he'd directed the action.

And he sharply decried those who testify against former confidants to ease legal troubles, bemoaning the longstanding practice.

"It's called flipping and it almost ought to be illegal," Trump said in the interview, adding he's witnessed similar scenarios over his decades in public life. "I know all about flipping, 30, 40 years I have been watching flippers. Everything is wonderful and then they get 10 years in jail and they flip on whoever the next highest one is or as high as you can go."

Suspects cooperating with prosecutors in exchange for a reduced sentence is a central feature of the criminal justice system, but Trump has long valued loyalty over legal precedent or practice.

Actually, there was a short period of time back in the late 90s where courts found that federal prosecutors' use of 5K and Rule 35 were illegal because it was bribery under 18 U.S.C. Sec. 201.  In other words, it encouraged false false testimony (which is very different than the claim that snitching should be illegal because its disloyal).

The main case was U.S. v. Singleton out of the 10th Circuit. Judge Zloch followed suit in U.S. v. Lowery, 15 F. Supp. 2d 1348 (S.D. Fla. 1998).  From his conclusion:

The Court's finding herein, although lengthy, may be reduced to one simple principle: it is not this Court's Constitutional function to engage in legislation. By invoking the theory that "settled expectations" demand exclusion of the Executive Branch from the reaches of Section 201(c) (2), the Executive Branch requests that this Court usurp the function properly left to the United States Congress. Moreover, the Executive Branch's theory amounts to nothing more than this: it has always been done this way, so it must be right. Such reasoning has never been the proper basis for a legal ruling and this Court refuses to make it the basis for one now.
Further, if, as the Executive Branch asserts, Section 201(c) (2) obviously excludes the Executive Branch, the Court feels compelled to question the need for legislation, hurriedly introduced in the wake of Singleton on July 15, 1998, to amend the Statute. See S. 2314, 105th Cong. (1998). By such action, Congress has reinforced the conclusion that Section 201(c) (2) presently includes the Executive Branch.
Finally, the Court notes that the testimony of cooperating witnesses and cooperating defendants is a valuable resource for law enforcement in the investigation and successful prosecution of criminal activity. Any exclusion, however, of the Executive Branch from Section 201(c) (2) must come from the Legislative Branch, and not the Judicial Branch. A judge's authority has limits and outside those limits or designated areas, the democratic institutions govern. Thus, if any changes are to be made to Section 201(c) (2), or if more appropriate, the substantial assistance provisions, it is solely for Congress, and not for the courts or the Executive Branch, to make them.
Accordingly, after due consideration, it is
ORDERED AND ADJUDGED that the Defendant, Oslet Franklin Lowery, Jr.'s Motion To Suppress (DE 134) be and the same is hereby GRANTED.

Singleton, Lowery, and other cases didn't last long.  They were all reversed.