Those three are indeed are used as power, but they are also part of the set {coal, oil, aluminum, uranium} that are used to fuel units.
2 is just too little IMO- you can have 2 planes per aluminum mine.
Iron and horses also give 2, but if we applied the consumption model backwards, would people be happy if they could have 2 swords per iron mine, or 2 horsemen per pasture?
Then why is it different for planes and helicopters? This is less of a meta game balance thing than a gameplay loop thing- the scarcity of aluminum doesn’t have any alternatives beyond “don’t have an air force.” Why do we let Aerodromes hold so many planes if you’ll never really be able to fill them?
I also think we really need to be able to trade for resource income, otherwise there’s just no stability at all. (and it’s almost cruel to sell the AI oil knowing they’ll build units they cannot fuel.)