The short answer is it doesn't matter what the law does or doesn not say: the employer (the restaurant) can tell them to stand as far away as it likes, since employment is "employment at will" in this country (unless they have written contracts guarantying their employment, which is unlikely). Among other things, employment at will means the employer sets rules at and for work--and those rules can includes going a minimum distance away before smoking. Employees who violate those rules can be terminated (or anything less than terminated: pay cut, suspended, hours reduced, etc.). So if you want your employees to stand further away, tell them to do so on risk of being fired if they won't.