The The Non-Libertarian FAQ is excellent (well, so far it’s excellent – I haven’t managed to finish reading it yet). An extract:
Do you think companies would voluntarily list that information even in the absence of government regulations forcing them to do so? If your answer is “yes”, how come large restaurant chains, which aren’t regulated, practically never list nutritional information on menus? It’s not that there’s no demand: voters in several American cities have supported laws to force restaurants to provide such information, so obviously a lot of people want it. Restaurants have just concluded that it’s more profitable for them to avoid it. I think they’re probably right.