They are not, they exist solely and explicitly to circumvent the "due process of law" particularly the 6th and 7th amendments.

How does a plea bargain remove the defendant's right to legal counsel or a trial? It does neither of those things. Yes, it's used erroneously by prosecutors and that's a huge problem, but the accused gets counsel even if they can't pay for it and the trial is always an option.

If you're saying plea bargains are designed to intimidate people into not going to trial, yes that's true, but that's not the same thing as circumventing an amendment. That's not the same thing at all.