(It seems like a pedantic point anyway; the fact that you're under no obligation to provide X does not imply that you don't have a policy restricting your provision of X. Plus, the comparison was to hospital smoking bans not e.g. cigarette vending)
There are probably several factors that influence what foods they offer and which results in their current general policies about what to stock. It probably makes more sense to first audit what those factors are and why they exist; otherwise, additional restrictions are more likely to force them into more expensive options since they'd have to obey the existing (probably ridiculous) constraints.