It's a ploy by the networks to get more people watching live. They know that TiVo/DVR users are far more likely to skip commercials (especially with the Hopper controversy!), so they do it on purpose to mess with scheduling in the hope that users will decide to watch live instead of timeshift.
Another trick is by using Twitter and getting the stars to tweet live during a show (even a drama).
"YOU! YES YOU! STAND STILL, LADDIE"
"How can you have any pudding if you don't eat your meat?"