Riverside County has a county wide CLEMARS system, and County wide ITAC systems. The purpose is for mutual aid. The system was designed with the intent of being able to either patch an external agency into the trunked system, or all parties could switch to either CLEMARS 9 or one of the ITAC channels. They also have VHF systems and UHF, so they can deal with external agencies operating on those bands.
It has nothing to do with encryption. This is one of many ways interoperability can be accomplished. RSO's current mutual aid radio systems date back almost a decade before encryption came along, and the predecessors to those systems date back over 20 years.
The hardware is nothing new... maybe actually using it is.