I would certainly concede that the previous posts were strong in their points about the challenges in the plea bargaining system. Perhaps, there can be something said about the idea of a prosecutor having to live with a workable and practical solution instead of an ideal one. The framers were wise enough to develop a system where the prosecutor has a burden to meet for different crimes. For whatever, if a prosecutor lacks the evidence for a criminal offense with heavier penalties, perhaps making a deal for something to ensure an offender pays their due penalty is better than not having anything at all to show for their efforts. While there are exceptions proving the frustrating nature of the plea bargain system, it seems to me that the system in place has met the overall needs of justice for some time now.
There are indications that the "snitch system" certainly has it problems in the courts of the United States. Perhaps the most egregious use of pleabargaining is the case of Salvatore "the Bull" Gravano, who was allowed to go free although he himself committed nineteen murders.
In the efforts to convict crime boss John Gotti, who became known as "The Teflon Don" because of his acquittals for major crimes, the FBI taped Gotti's headquarters where he spoke of making hits on people. When the tapes contained pejorative comments about his right-hand man, Gravano, the FBI played these tapes and induced Gravano to break Omerta and testify against Gotti. In return for this testimony, Gravano went into the FBI Witness Protection Program. Later, he returned to a life of crime as a major drugdealer, along with his family, in Arizona--but not before writing a book, of course!
While there are arguments that prosecutors can get the "bigger fish" by catching the "small fish" and making a deal with them, there are certainly flaws in this system as indicated in Gravano's history. Another flaw is the fact that the "small fish" may lie in order to save themselves. There are documented cases in which these criminals have fabricated testimony that leads to the conviction of the "bigger fish" that prosecutors want. Unconscionable prosecutors do not concern themselves with this situation--even at time encouraging it--since they win their cases, make a name for themselves, and gain monetary rewards.
This pleabargaining system has not always been in place in the judicial system--at least, not to the extent that it is today. There was a time when the criminal was caught directly.
I do not think that we need any laws to keep defendants from plea bargaining in any given situation, if that is what you are asking. Here is why I think this.
It is the prosecutors who generally decide who they will allow to plea bargain and who they will not allow. They tend to decide this based on how sure they are of being able to convict a person on a particular charge. They usually go for the most serious charge that they think they can convict a defendant on.
The system is set up in a way where the prosecutors are in charge. This means that it is not like the defendants are getting away with something. Instead, they are typically getting as much punishment as the prosecutors think they can "win."
So I think the system is already against defendants so there is no need to pass laws to keep them from getting away with stuff.