Well if the DA believes they can be tried for other offences the DA should argue that.
Under the current system, it seems to me that plea bargains act as contracts "I agree to serve X years & not go to court", despite being phrased as "I admit guilt for Y crime" on paper.
Of course people actually convicted of crime Y also exist. I'm just surprised that this whole thread assumes that only they exist.
It would be trivial for a police officer to carry a bag of marijuana/ cocaine / amphetamine and arrest anyone they like for possession.
1. https://www.nytimes.com/2018/01/24/us/baltimore-officer-vide...