The standard is that if the prosecution intentionally "cheats" and obtains a conviction through unconstitutional means, the conviction cannot be appealed unless it can be shown that there was a "reasonable probability" that the verdict would have been different without the violation of rights. Worse, in our adversarial system, prosecutors are essentially obligated to argue that any misconduct had no effect. One might even conclude that they are "obligated to cheat".
Ken White, a former prosecutor and legal blogger at Popehat has an excellent explanation of this in his recent piece "Confessions of an Ex-Prosecutor". I'm tempted to just quote entire sections from it, but perhaps better just to link to it. If pressed for time, start with the section "Prosecutors Are Duty-Bound to Argue That Rights Don't Matter": http://reason.com/archives/2016/06/23/confessions-of-an-ex-p...
So, while your point is excellent, one might ask whether our voting system should be more like an athletic contest, or a legal proceeding? And if the answer is "like an athletic contest", what does that say about our legal system?