Nobody tried to make P25 listening illegal...
What you are trying to do is illegal, because as already mentioned, your radio HAS to transmit on the system to connect. And that transmitting is illegal on MPSCS frequencies.
What these folks are trying to tell you (I assume in utter good faith), but which you may not be getting because you don't have the background they may be assuming you have, is this:
With older Motorola trunked portable radios (e.g., MTS2000), it as possible to put the value "None" in the parameter "Affiliation." Doing so pretty much accomplished Receive Only function, and (putting aside any software license issues) was perfectly legal.
For reasons that are not entirely clear, with the XTS series of portables, "None" is no longer a valid value for the "Affiliation" parameter. Rather, that parameter is fixed at "Auto." This means that every time you turn on your radio selected to a trunked system talkgroup, every time you change from one channel to another trunked system talkgroup, and a bunch of other times, your radio will transmit (entirely on its own), sending a data word that contains the SysID, your radio's UserID, the Talkgroup ID on which you are selected, and a request for affiliation. This is where the unlicensed and impermissible transmission comes into play.
It is, in substance, a shortcoming of the XTS line of radios (at least, a "shortcoming" from the perspective of trying to make them into RXO scanners).
Now, a lot of folks who don't do radio for a living aren't aware of what I'm describing to you, and for lack of that awareness, can get themselves into trouble. The responses that apparently offended you were simply trying to warn you of what appeared to be an unrecognized danger.