The reason (fair or not) to limit calorie/nutritional information requirements to chain restaurants is that requiring it of every restaurant for every item would really place a really large burden on small establishments. It isn't costless to get this information, and aside from monetary cost there's a time cost. I wouldn't want my local French BYOB to have to send every new proposed menu item to the lab before putting it on the menu.
It's more reasonable for large chains because their menu items are standardized and the cost can be spread over their entire chain. And, more than that, the major chains at least already know the nutritional information about their products, they just don't necessarily share it very prominently absent regulation.