The criminal activity could have been directly related to forcing them down the hill, for example brandishing a weapon (in and of itself a crime), impersonating LE if that was his ruse, even just the abduction itself could have been the aforementioned criminal activity about to begin.
IMO they don't want to reveal these details because they are planning to use this information during interrogation of POIs AND witnesses. If a suspect says, yeah I was there and talked to them but I said this, not that - well, they have the recording. And no one but LE knows for sure what was captured.
It is also a check against which the accuracy of witness testimony can be measured. If someone calls in a tip and says "my friend did this. He told me that he kidnapped two girls by saying he was a cop." But LE knows, from the recording, that a different ruse was used - they know they aren't getting good information.
And at a trial (if there is one) they also want to eliminate the defense of "my client was coerced to say this because this information was already out there in the press." Alternatively, you can imagine a scenario where a defense attorney can cast doubt on prosecution witness testimony by saying "this information was already well known before this person came forward to police because it was reported by the press."
All JMO.