Personally, I think people overestimate the calories required to cycle and so they probably end up eating in excess of what they burn. If you're burning 300-500 calories per hour and then loading up on pastries and such, then no, the weight isn't going to come off.
As for cycling not being good exercise, I'd challenge some folks to do 10-12hrs a week of structured workouts like I and many more fitness/competitive minded cyclists do and then reassess whether they think it's good exercise or not.