Firstly, those "rice" paddies aren't stagnant sitting water. Water flows through them from one to the other and are bigger than some people sometimes think.
Two, Southeast Asia stays fairly warm.
Three, just because they'll live like that does not mean that they should. A neon could probably live in 68F water but that does not mean that is where they are in any way comfortable. I could survive in 45 degree weather for quite some time - but I would be pretty miserable.
Four, no tropical fish "needs" a heater if you can maintain a steady, appropriate room temperature all of the time. But if you cannot, they need a heater.