That was an intentional design of the system. There were a few different modes of operation for the system:
NOR = Normal = Comm Center to Car only (one car at a time) - In this mode, you'd only hear the PCC.
TAC = Total Area Coverage = Many cars together on one "channel" used for incidents - In this mode, you'd hear both sides.
SIM = Simplex = Radio to Radio
COM = Provincial Common = 142.770 Repeater
AUX = Auxiliary Channels, used for non-OPP repeaters in specific setups. For example, OPP in Northern Ontario had access to MOH PCOM 150.1000 to coordinate helicopter landings via the AUX channel.
They could also do a "Line Patch" which was used to allow two cars to talk to each other. This would use the A and B channel for each respective car.