I think cabotage rules do not allow foreign carriers to operate domestic sectors in most instances. So Air Asia Malaysia couldn't fly to Bangkok then onto coughet (and pick up new pax in Bangkok). Hence why Air Asia Thailand may have been used. As i said, it's complicated!
I think we're in violent agreement on this, essentially saying the same thing. But cabotage is not the reason that the Air Asia's and Air Asia X's are different entities within each country (ie if Air Asia Indo can fly A320's between AU and ID, why wouldn't they also - if they had the type on their AOC be able to operate A330's as well. There would be other reasons I assume.