My guess is HP makes such an enormous amount of money already from movies, games, toys, and other tie-ins, that they can't be bothered to chase down the odd digital infringement of a plain text copy of the original books.
I'm sure the scripts of Star Wars would be similarly ignored if they were used.
I think Microsoft employees should be smart enough to figure that "a globally beloved collection of seven books" (their words) is probably not free to use.
It is just very hard problem when you are very popular work. Trying to find and track and take down all copies of certain work online is constant fight. Sometimes things just slip especially if they are not that popular.
Something like Harry Potter might be shared every day. And I mean as pirate work distributed as new copy. Staying on top of that will be very hard work.
I'm sure the scripts of Star Wars would be similarly ignored if they were used.