I suggest reading the book 'Botany of Desire'. One chapter is dedicated to apples, I'll try to summarize from what I remember.

The popular types of apples you eat all the time come from grafted trees. When you plant an apple seed it's a crap shoot as to what type of apple you will get. Usually it's a crab apple. These are tiny and the taste can vary. Occasionally, you get an awesome apple. This was like striking gold. You can reproduce this tree by grafting parts of it onto other apple trees and grow more awesome apples.

Knowing this fact, it means that Johnny Appleseed was planting trees that would bear mostly crab apples. Crab apples aren't good for eating, but they are good for making cider. Johnny Appleseed was bringing alcohol to the frontier, not Granny Smith's.