Each 2.4 ghz transmitter has a unique GUID or Globally Unique IDentifier. Its a code made up of x number of binary bits. For example if its 32 bits long there are over 4 billion possible combonations (2 to the 32nd power), this GUID is imbeded in the firmware of the radio. (network cards are coded the same way coincidentally)

When you bind the reciever to the transmitter you tell that reciever to memorize that GUID in its nonvoliatle (remains even if the power is off) memory and only accept signals that start with that GUID so even if you had another radio on that channel it would ignore the radio since the signal wasnt prefixed with the correct GUID.