Microsoft earlier this week sued (PDF) Barnes and Noble, which uses the Android OS in its Nook e-reader. Microsoft is alleging that B&N violates patents filed dating back in 1998, including patents related to capturing, editing and remote retrieval of documents. The patents under dispute are listed on Geekwire's site.

Many lawsuits have flown between device makers for using Android. Apple has sued Motorola, Oracle has sued Google, the lawsuits are becoming hard to track. Many observers have analyzed the cases, and Florian Mueller of FOSS patent blog said Android indeed violated Oracle patents after sifting through source code.

But Groklaw is asking why hasn't Microsoft sued Google directly?

" Well, this *is* Microsoft. It's jungle ethics. Don't go after the strongest in the field."

"What they seem to want is for folks to quit using Android in their products by scaring them with litigation," Groklaw writes on its blog.

Good point. The strategy is simple. Carriers may be wondering whether it would worthwhile to chance on Android, a free OS, or pay a billion bucks to license Windows Phone 7, which is the only other alternative.

Companies would rather take a chance on Android than be stifled by WP7, which could be limited in scope. Nokia perhaps opted for WP7 with a premonition that cases would be filed against any Linux-based mobile OS such as Android or Meego.

Google wasn't quiet either. They came out slamming Microsoft, saying that the lawsuit would hurt innovation.

This also begs the question: what happens to Android with legal threats flying around? Nothing. Lawsuits for Linux were filed by the dozens, but the OS went on. Android will not be shut down -- it's too widespread in popularity to pull the plug on the OS.

Android may also be in violation of GPL for its headers, points out ITWorld. In response, Torvalds did not seem concerned about Google messing with headers.

Back to Microsoft, what exactly is the company's modus operandi behind suing B&N? It clearly needs to stem Android's growth so it gives Windows Phone 7 some breathing room to grow. WP7 has not done well, and it's never easy to go after Apple, if Microsoft intends to take on iOS.