The motivation can come up discovery though. If there is something nefarious, you would expect the defense to bring it up during pre-trial motions (which are public, just not presented to the jury). The defense also has wide lattitude to bring up such issues to the jury (although they are often limited in what they can argue about it).
In some cases, selective prosecution is itself a defense; but (as you allude to) that is a very high bar for the defense to clear.
The actual cause of the investigation is unlikely to come up during discovery or the trial. This is because of Parallel Construction [1]. Here is the first paragraph of the Wikipedia article to explain:
> Parallel construction is a law enforcement process of building a parallel, or separate, evidentiary basis for a criminal investigation in order to limit disclosure as to the origins of an investigation.
Parallel construction is about hiding methods, not motives or the actual transgression. What you’re describing is simply abuse of power: the cops don’t like the beat you’re reporting so they start ticketing you for going two over the speed limit.
I would argue that hiding methods is one way to obscure motives of those building the case against a defendant.
The specific discussion was related to a "wait-and-see" attitude in regards to the motives of the justice system of France (not the motives of the alleged perpetrator). The suggestion was that if this was politically motivated then that motive would be revealed once the discovery process began. However, if the French legal systems wants to hide a political motivation then they can use parallel construction to hide their methods, thus obscuring their motives.
Also, I didn't "describe" anything. I pointed to a Wikipedia article that, in part, declares parallel construction as supported by the Supreme Court of the US. So, at least in US law, it is not at all an abuse of power but rather a totally valid approach that law enforcement can take to build a case against someone. The entire reason I even know the terms is because it showed up so often in the TV show Law & Order.
I guess you have a point, but I think the specifics depend on jurisdiction.
That said, the defense can say whatever they want about what they perceive as selective prosecution... but how do they know? It's not like they would have more knowledge about the prosecution's thought process than the average conspiracy theorist?
In some cases, selective prosecution is itself a defense; but (as you allude to) that is a very high bar for the defense to clear.