The Prenda modus operandi doesn't seems much different than what the US Police is doing, so why do they face prison ? Don't get me wrong I think they deserve to go behind bars, but I would have thought what they did was legal in the US.
A lot of the trouble here is how they went about this scheme, not merely the basic idea of what they did. They repeatedly lied to defendants and courts about their operations. Some of the materials they sued over were things they didn't actually have copyright for, but they used false documents to claim that they did. Those false documents involved forged signatures and even invented a person who didn't exist. They even filed defamation lawsuits against a person whose signatures they forged, for daring to say that they hadn't signed this stuff.
Wikipedia has a huge list of the crazy stuff they've done: https://en.wikipedia.org/wiki/Prenda_Law
So, it's not just about trying to entrap people by "pirating" their own content. It's about massive fraud and abuse along the way.
There are a lot of reasons, the most critical one being that it involved multiple levels of misrepresentations to the involved courts, which is not part of a normal law enforcement sting operation.
(There's also a difference between a criminal sting operation where the prohibited offense is, e.g., buying the goods being offered vs. a civil copyright "sting operation" where the prohibited offense is "copying some work without the consent of the copyright owner", and the active assistance of the copyright owner is involved in the act of copying.)