Kings Indian Attack - French. 3...dxe4?

So I have been looking at games that started like this. In particular there is a beautiful game played by Fisher that also started like this. The KIA set up that follows with the N on d2 going to f1, h2 and g4, and the potential sacs to open the h file look so fun to play. 

However, while looking in the opening explorer, instead of leaving the tension, I see a common response from players is to exchange the pawns early on move 3. So I was looking for general ideas, but this move is not really discussed. Probably dubious on high level play, but that does not mean much to my level. My own thoughts were to just focus on development first (as always), black probably has to play e5 at some point which in combination with c5 or Nc6 mean the d4 square is probably for white to take. If I am allowed to play e5 myself, I might go for King side attack anyway. Does this makes sense?

Anyway, why would you be happy to see the exchange on the third move?

Exactly the reason why 3...dxe4 is unpopular at high level. Moving the e-pawn twice is a loss of tempo.