This couldn't possibly be because they misjudged demand. A company like Apple is a well tuned machine. There is a calculated reason for doing online only sales in the beginning. I honestly can't think of why, but clearly they were considering something in doing this.
EDIT: actually, now that I think about it, they may have known that despite all the hype they've tried to create, demand will be underwhelming. And what better way to demonstrate a lack of demand than a lack of lines at Apple stores. By going online only, they can still claim to be doing very well without having to deal with very obvious signs of a lack of demand.