1) eReader owners want ebooks, not print books
2) pricing the ebook version of a new hardcover at $9.99 doesn't cannibalize print sales in any significant way
3) eBooks need to come out at the same time as the print to take advantage of any buzz
So really, for Dan Brown, it was 100,000 ebooks in hand, versus an unknown number that might be hiding in eReader bushes. The two million mark for the hardcover might be said to make this seem like small potatoes, but that is still a heck of a lot of potatoes (I am mixing metaphors, I know).
And as of now, TLS is still number 1 in the Kindle store, ahead of a lot of free ebooks.