The liability question (Google offering patent indemnification to Android trademark licensees) is a different issue. While in an ideal world, Google would be able to provide such protection to its licensees, it is simply more expedient for Google to vigorously fight Oracle's claims in court and provide protection to its partners indirectly.
Concerning cost/benefit ratio, Oracle wouldn't have to sue them all. It could contact many of them and some would pay. Some would refuse to pay, and then Oracle would single out one or two or more of them and sue. Just the fact that they make good on their threat to sue would get some others to pay. And if they prevail over only one, they will get everyone to pay.