The room was packed, and the audience was laughing almost constantly.
The movie isn't a comedy. They are laughing because it's so bad.
I feel inclined to agree with you. My date, 31yr old woman, asked me not once but twice why there were giggles and snickering. She was enjoying the movie.
There was an 11-12yr old girl about 3 seats down and she was giddy the whole time. My answer to my date was that some were likely snickering cause of the cheesiness and some were giggling, like the 11yr old just down from us, because it was just what they imagined.
Personally I was laughing on the inside for just how bad the acting and dialoge was.