my understanding of it:

AudioChannel is for writing a fixed sample size on the disk. So in order to use an oscillator, you create an AudioChannel with a fixed length, generate samples and write them into the AudioChannel, then play() the AudioChannel. Not good for a continuous stream of sound.

AudioStream, instead of a long fixed size, repeatedly calls its audioStreamWrite() method, in which you fill its buffer with a short bit of audio, which is then passed to the speakers. To use an oscillator here seems to require a bit of bookkeeping (updating the phase of the oscillator each time...)

hope that helps...