If a prosecutor starts trying his case in public (as they often do) by speaking to the press and the media, the citizens get a one sided premature view of the strength of the prosecutor’s case. This means that many folks called to jury duty have already formulated a preconceived notion about a defendant’s alleged “guilt” before they get selected and before they hear one word of evidence. This can undermine a defendant’s right to get a fair trial.
It isn’t that much different if a defense team engages in some effort to similarly taint the prospective jurors, just from the other direction.
Just as a defendant has a right to a fair trial, the prosecution has a right to obtain an impartial jury. And courts are supposed to help make sure that the “playing field” is relatively level.
When a major crime takes place, most of us feel a sense of relief when the press announces that an arrest has been made. But just because Jimmy Bob has gotten arrested doesn’t mean that the “right man” has been caught. We can’t always avoid our reaction to the news. But that does raise the risk that whoever is chosen to be a “fair and impartial” juror may not have already predetermined the outcome.
That’s part of what we mean by “tainting” the jury pol.