In my opinion, they are creating a market for webOS with the Touchpad as a loss leader. What they intend to do with that market is anyone's guess, but if you think about it: it will be much easier for them to get other hardware makers and software vendors involved when they have hundreds of thousands of users out there, many of which will grow to like webOS and will want to buy new hardware when their Touchpads need replacing in 1-2 years time, as well as making app purchases which have already shot up according to the webOS devs I follow on Twitter.
Of course a few Touchpad owners will be technical enough to install some form of Android on their tablets and there are already efforts under way, but in my opinion these will be the minority. The majority will stick with webOS, giving HP a valuable user base to build on.