That might negatively affect sales. Many people might want an app for a specific need, and once that need is gone, will never use the app again. Maybe instead of a free trial on a $1 app, it could be something like a 20 cent trial, and if you buy the app, you are credited your 20 cents.
The developer has control over the pricing model though right? For an app that solves a specific problem one time and then is useless, it would be foolish for a developer to offer a free trial. As far as I know Apple does not force free trials.
