This cannot be applied to the low cost AirAsia flight. For this low cost you definitely have to buy. Anyway, they will ask what you want to buy, though.
Indeed, I do agree with the author of this topic. Laotian is not that stupid.
It's very old Thai movies (over 15 years ago) base on real story and the author was work as a flight attendant for Thai Airways and she has many stories many incidents to tell you through the books and films.