CBS must know the whole Ashton Kutcher thing is a big risk. They also must know that no one wants to watch him every week. I think they got suckered into this and are prepared to suffer through 26 episodes of really bad acting. But, the good news for CBS is apparently they only signed Ashton to a one year deal. I don't think it really matters to Ashton though. Even at the discounted rate per episode he is still going to pull in almost $20M for what amounts to less than 13 hours of television.
CBS was kind of stuck since they had already committed to next season. I guess they thought Ashton was their best move. I think they should have just brought in that monkey from the Hangover movie.