The reason everyone can do Earth walls!
Everyone assumed that you have to have Earth affinity to be able to use Earth style techniques.
Well, as far as I'm concerned, it was never stated this way.
Think of Kakashi - he can do Raiton, Doton and Suiton. Yet, it was stated that Raiton is his affinity - not Doton or Suiton.
Why is that?
Because most likely every ninja can only have ONE affinity and can only learn (with talent, hard work, or both) how to use the others.
That's why in this chapter it was clearly stated that the Earth wall done by everyone will be pretty pathetic - they don't have affinity to doton nor did they ever learn how to use it.
That's my opinion on the subject.