Can my employer demand I pay back money stolen from their lunch wagon?

Unfortunately, an employer can make an employee pay for money that was stolen out of the "till" (i.e. cash register, etc). In an "at will" employment arrangement an employer can hire or fire as it deems necessary and impose whatever terms and conditions of employment as it sees fit. Unless this violates a union or employment contract or this action is the result of some form of discrimination, an employer is within its rights to require such a payment. 

That having been said, an employer may not deduct from an employee's paycheck for this; at least without the employee's prior consentor if an existing company policy allowing for this is already in effect. If such a deduction is made the employee can report such action to yourstate's department of labor.