i didn't say anything about the default airport
the frequencies are loaded by default for whatever airport you choose to start at... this all comes from the data in the apt.dat or similar...
remember, many folks just want to play a game... they want to just jump in and go flying with everything working as if they had done it themselves... if you don't want your craft to pop up with the radio playing, then someone has to adjust the craft to allow for starting with the radio off or forcing it off all the time or the same for the volume... some might even force the radio off and to start at the lowest frequency the radio can handle... some might have an option to retain the last frequency and have it in place when starting anew... i remember the c172p going through the steps to do this and be able to save or not for the next start...
remember, not all craft have the same realism standards or settings... many, it seems, go for the FDM being the first realistic thing and then maybe the model and animations... eventually, they might get around to the external world realism stuff like the radios and such...
[EDIT]
i forgot to suggest that maybe you can do what you want via a property? --prop=/sim/radio/volume=0 or something like that... you'll have to find the actual properties, though... remember that there's two radios in may craft so there may be more than one radio volume property to be set...
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."