Why is HTC being sued and not Android - Because HTC is an corporate entity that makes money off Android - Android isn't a corporate entity on its own - who would they sue if they wanted to sue Android?
Google develops Android and markets it, and I would assume that they could be sued for developing an infringing product. Google most certainly makes money, and a lot of it, from Android.
But Google makes no phones on its own, unlike HTC and Samsung, so Apple goes after those companies to keep the devices from being imported into the US. And Apple can fight Android in a legal proxy war rather than directly suing Google.