Actually, the subchannels use what is called a subaudible tone. If the radio that is transmitting does not use the same tone that you are using, then your radio never opens the squelch (this is the same way that HAM repeaters work with PL tones).

Yes, anyone with the same subaudible can hear you and transmit to you....it is not really a privacy thing, but it allows more people on the same frequencies.