Why does katniss drug Peeta before going to the feast?

In the book, Katniss recieves a sponsor gift containing sleeping syrup. She dumps the syrup into Peeta's soup and causes him to fall asleep while she goes to the feast.

In the movie, Katniss goes while Peeta is sleeping. She did not give him any sleeping syrup. He fell asleep naturally, and she left to go to the feast.