I'd be willing to bet they bought it for Web OS and the patents.
HP will be able to produce a tablet with an OS no one else has, and have a product that's a bit different than anyone else's.
From a retailer/manufacturer's POV, Android is a PITA, how do you make your product different if everyone has the same OS, Windows commoditized PC's, and Android threatens to do the same thing with mobile devices.
Don't get me wrong, I love my Swiss army knife that is Windows, and from my POV, commoditizing PC's is a good thing, as it makes stuff inexpensive for me to buy....