Always surprises me how much the "land of the free" allows corporations to control their lives. How can a company claim the right to decide what a person can and can't do after they have left the company?
It's called contract law. When he started for working for Microsoft, which employed him and paid him well for years, he willingly signed a piece of paper that said that he could not immediately go to work for a directly competing company - at least, one of the listed ones - for a specified period of time. As in, he couldn't jump ship, immediately sign on to a competing company, and say "Ok, guys, here is what my previous employer was doing, and here is how we undermine their confidential plans and strategies."
When reading that, focus on the "willingly signed" part. In this case, the "land of the free" is saying "If you sign a legal contract saying you will not do X after you quit, you can't legally do X." I have to ask you, seriously... Is that so terrible and unfair?