Lee Child addresses this himself on his own
web page. It says:
"Lee is thrilled and can't wait to see the final cut.
Don't forget Lee has spent equal time on either side of the page/screen divide and understands better than most that changes are necessary and inevitable. And like Reacher he doesn't favor half-measures...if we're going to have a movie, then let's have a movie star in it! There is only a small handful of genuine global male stars. All are great actors—you don't get to be a star if you don't have the chops—but Cruise might well be the best of his generation. And again, Lee has been around actors forever—Laurence Olivier was still in his sixties the first time Lee worked with him—so he's a pretty good judge.
Plus, from an author's point of view, it's more exciting to see a new and fresh vision. Lee doesn't want to see a literal translation. Without comparing himself to Bob Dylan, he wonders what Dylan wanted to hear when someone covered, say, "All Along the Watchtower"...a tribute clone, or Jimi Hendrix? Lee thinks the Cruise/McQuarrie interpretation will build on the books in the same way."