Telling 10,000 employees to move to Redmond isn't particularly practical or likely. Particularly when they could likely find jobs with other Silicon Valley companies instead.

LinkedIn is not owned by Microsoft yet, so it can't base its decisions based on this fact.

microsoft has a history of leaving large acquisitions where they are. skype (global), powerpoint, hotmail, yahoo ads, & webTV (mountain view), yammer (sf), dynamics (north dakota, but grew in redmond), an app virtualization thing (boston)