Surely it’s because by law if you were not part of the vote then any action you took would be “unprotected”. If that were the case Jetstar could sack you. So AIPA are assisting their members in not getting fired. Isn’t that the basic job of a union??
Whether AIPA should also poll for PIA is a separate and valid debate but explaining the law to your members so they don’t jeopardise their careers seems like a sensible move.