How is the Band in a Box plug in able to work with Pro Tools but only available with the Windows version ? Pro Tools is for Mac AND PC with the SAME features on both operating systems.

The plugin is just the front end of the Biab app(Mac)/exe(Win)
the plugin communicates with a Biab app or exe running silently in the back ground,
it asks the Biab app/exe could you please generate up this RT No. @ this bpm and these chords then save it in this folder,
that track then appears in one of the tracks in the plugin,
the plugin can then play back that track directly from the folder in sync with the tracks you have in the DAW already,
if you like the track, as you drag it from the plugin into the DAW track it is just dragging it in from that local folder,
if the track don't fit hit the Generate button for another.