It is how the business is now.
Music star gets big, and makes a movie to score more money. Their fans will most likely see the movie, and it will create enough buzz to get more people to watch it.
I have not watched the movie yet, and some dumbasses are saying it can rival "Grease". Those people also need to be shot repeatedly in the chest.
On the same note, I honestly thought that Eminem is a decent actor, but not a great one. He is better than Britney Spears and Mandy Moore, but cannot rival Jennifer Lopez's performance in Selena. By the way, I think J.Lo went to the shitter after Out of Sight...