Copyright trolls not being able to upload chunks seems like a myth along the lines of "if you ask a cop if they're a cop, they have to say yes". It's easy enough to create a separate legal entity that doesn't have any rights to distribute a work, and then sign an indemnification agreement for any copyright violation that happens in the course of investigation. And if you wanted to be real paranoid, mod the client to never transmit say 20% of chunks, so even if some court finds that participating in a swarm at the behest of a copyright holder is constructive distribution, that last 20% is still actionable.
Even if this is true, there are several difficulties with this approach, you'd need to figure out a way to refuse clients from countries where you have exclusivity deals and aren't allowed to distribute, which would quickly be noticed. Besides, if the problem got big enough, tracker staff could require users to seed a few different torrents from different studios before having their accounts fully unlocked, and studios would never seed others' copyrighted content. Sure, you could defeat that with studios having contract between each other and so on, but that's yet another difficult problem for them to solve.
The risk and effort is probably not worth the reward, considering how many public tracker users are there.
You seem to be thinking that movie studios can only operate as singular entities and in system-legible ways.
What I'm imagining: someone who is mildly connected to execs at various studios/labels starting a company that participates in private trackers, and then passes information about infringement/infringers onto studios. They would only need one or two studios as clients to prove the concept and (informally) prove the idea to the rest of the industry. Their agreement with client studios includes an agreement that they won't be sued for infringement that occurs in the process of finding (other) infringers, doesn't include any license to works, and certainly doesn't include the ability to sublicense!
Sure it's possible that when this eventually goes to court, a chain of "activist" judges might go against the status quo of a company taking steps to protect its "property" - discard corporate veils, call the investigator's uploading an implicit sublicense, etc. It's just not likely, and the failure mode still would be individual licenses for the specific downloaders that were in the swarm at the time, not blanket rights to redistribute indefinitely.
