AirAsia has a "no outside food and drinks" policy.
Apparently, AirAsia is imitating restaurants. If you walk into a restaurant but consume foods and beverages you brought from outside, the restaurant will lose revenue.
But AirAsia is not a restaurant. Why should it enforce the "no outside food and drinks" policy on the passengers? We have already paid the air fare!
And I believe, if I purchase a bottle of mineral water after airport screening point for consumption on board, I haven't breached the aviation regulation…
Q: Can I take an empty water bottle and fill it up at the airport?
A: Yes, you can take an empty bottle, and fill it up once you have been through security screening. Please note that you will not be able fill up your bottle prior to the security screening point. You will still be able to purchase ready filled containers of water after passing through the security screening process.
Source: Flysmart FAQ