That's how they operate,..
While it may seem odd.. I understand it..
By avoiding the whole "cancel", "re-sell" etc, etc. etc. issues, they also sidestep all the necessary internal policies, procedures, accounting, programming, and other "stuff' that goes with having to manage, re-manage a constantly shifting inventory.
They've chosen- and I see why-- to adopt a simpler "one and done" booking strategy in that seats are sold only once and by having a nearly 100% no-cancellation policy in place-- thus no need for, nor opportunity to "re-sell them", and also by not having many "connections" (on AirAsia called Fly-Thru and only available on selected sectors) they've essentially made their whole inventory management system much more simplistic to administer.
Essentially, in most all cases, when a seat sells-- that's it.. There's no re-booking or cancellation risks that they've got to manage and also "price" into their fares like major carriers do by the use of their myriad of different fare classes..
For a carrier like AirAsia, who has gone for the 'streamlining" of the operation, I get it and for them, based on what they are, where they are and who and where they fly, right now, it makes sense in the larger picture.