Their latest license agreements allow them to probe your computers and upload software at will, which is causing problems for banks and hospitals, who are required by law to restrict access of third parties to their clients' private information. Indeed, one banker who raised questions about this was apparently told that Microsoft "plans eventually to eliminate users' ability to disable Microsoft's access to their systems".
They're not the government. They don't need no steenking search warrants. Hard-core libertarians, if you want to know what a legal system dictated by the unfettered marketplace looks like, this is it.