American English is English. It's at least as standard as British English. (You can argue this point linguistically if you like, but it is clearly Duolingo's practice to accept American English as a least a standard, if not the only standard.)
Pop/soda is probably the single most famous regional variation in American English--it's something every secondary-school student is aware of. If the goal is to teach people to actually communicate with other people, this is at least one variation that does need to be included!
I'm from South Africa and was wondering about "cold drinks"? This is what we would call anything drinkable other then water or alcohol. Coke, pepsi, Fanta, Cream Soda, even mixes like 7up or Oros? Does this count as any one of those or simply that of said "fizzy drinks"(anything containing carbon gas like Coca Cola)?
So.... 'King of pop' sounds reeeeally funny to some americans? And listening to pop would look really funny holding a glass of carbonated beverage to your ear? </troll> I know, words have different meanings in different parts of a country, state or even a city, but since nobody brought up this aspect, I did. =)