Correct me if I'm wrong but the idea was never to let a foreigner leave their vehicle or rig in Mexico with the owner absent. The idea was to make sure that the unit returned with the owner.
As far as I know and I stick to the rules of aduana, it has never been legal to leave your rig in Mexico without permission from the local aduana office which includes leaving your rig on one of their properties until you return.
What is the purpose of the TIP permits anyway? It is to keep people from arbitrarily leaving and possibly selling their vehicles under the table. I realize most think, "well, that's Mexico" but we do have laws and the logic of "when in Rome" sometimes isn't a show of one's home country cultural traits.
These issues are exacerbated by foreigners who seem to try and find a way around things like in SMA where they issue license plates under the guise of a farm workers union. The list goes on.
Anyway, there's a reason for the permits besides making money and they continue to tighten the grip as there are more and more offenders and I am not referring necessarily to rvers