This should not be surprising. According to the Farlex Book of Grammar's Mood section:
We usually use emphatic do to stress the fact that something is the case.
Courtroom proceedings are largely concerned with doing two things:
The first is settling questions of law to determine how the law might apply to a certain case, and the other is determining questions of fact to determine if the relevant laws have really been violated:
question of fact n. in a lawsuit or criminal prosecution, an issue of fact in which the truth or falsity (or a mix of the two) must be determined by the "trier of fact" (the jury or the judge in a non-jury trial) in order to reach a decision in the case. A "question of fact" may also be raised in a motion for summary judgment which asks the court to determine whether there are any questions of fact to be tried, allowing the judge to rule on the case (usually to dismiss the complaint) at that point without a trial.
(Gerald N. Hill and Kathleen T. Hill)
The U.S. political system was founded on the basis of fear of unjust government, and not just from the executive or legislative branches. A courtroom known as the Star Chamber is quite infamous for violations of basic human rights such as overly long proceedings, ordering inhumane punishments and most relevantly unfair trials..
As such, there is a strong emphasis in the U.S. legal system on avoiding the conviction of the potentially innocent. According to the C.A.T.O. institute's webpage on the Blackstone Ratio, William Blackstone wrote, "[B]etter that ten guilty persons escape, than that one innocent suffer." and Benjamin Franklin increased that ratio by tenfold, by writing that "it is better a hundred guilty persons should escape than one innocent person should suffer." The basic principle here is that a powerful government run amok can cause so much more damage to the general populous than even the criminals, so judicial proceedings need to be handled with the appropriate care as to not harm the innocent.
In criminal cases, the standard is that a suspect is presumed innocent, until their guilt is not only proven but proven beyond a reasonable doubt. If there is even a slight chance that the defendant did not take the bag, then it is possible that this standard of evidence is not met. Civil cases have a lower standard of evidence, but even then the emphatic mood can still still helps to indicate that the necessary standard of evidence to determine a matter of fact has been exceeded.
This is probably particularly important for when a case is appealed, and a higher judge has the opportunity to review the case. By using the emphatic mood a courtroom emphasizes that this is not a disputable opinion, but a finding of fact that not only satisfies, but exceeds the standard of evidence required by the case, so as to justify the verdict and penalty, reducing the odds that the finding will be remanded back to the lower court or overturned.
So my principal hypothesis is that expresses that nothing was left up to chance. There is also a certain rhetorical appeal to emphasis, which is why hyperbole exists, but that applies to just about every subject irrespective of nature.
Aside from that, once a lingual practice has been adopted by the courtroom, it is very difficult to cease. Plain language initiatives often leave matters open to interpretation that were previously settled through precedent and left alone due to the principle of stare decisis.