Here's what I think: Ben kept his word. He let Mike and Walt leave. And he didn't worry about Michael talking when he got off the island because then he would admit to killing two people. Besides, Michael got back and found out that he was really dead, along with all the other Losties, and his body was at the bottom of an ocean trench. So....Ben's people contacted Mike and said:

"Your son's safe, but don't you feel really bad about what you did? Wouldn't you like to try and make it up to your friends? Leave Walt here at spooky kid day-care and go help the people you betrayed because if you don't, Charles Widmore is going to kill them and open up his version of Fantasy Island where everyday a midget yells 'Da Plane, Da Plane' and Oceanic flight 816 breaks in half directly over the log ride. Oh, by the way, your new name is Kevin Johnson."

Let's face it, if Ben can get Sayid to be an assassin, he can get Michael to be a janitor.