content: It's intentionally bad in order to draw attention to him, clearly establish his role, and because he's in training so is not very good at all the Jedi powers yet.  You can't have the reveal at the start, because the telekinesis is sure to elicit a bigger response because there won't be an indication that anyone else is "in on it".  If the cashier played along it'd be obvious right off that bat that he's playing jedi, instead of getting those "what the hell, Jedi's are real?!" moments.