The option isn't taken away. They can go to the grocery store if they want. That's still an option. There's no cognitive dissonance. As an institution, a university is not required to provide every option to its students. It, similarly, shouldn't bar students from making choices they want to make.

EDIT: And if they make choices that turn them into unsuitable students (poor grades, arrests, whatever), the institution is free to suspend them as appropriate.

This is a trinary variable, not a binary one. Sure, between the two options of giving people freedom to make their own mistakes versus applying coercion for their own good, I'm in favor of the first option. But there is a third: soft paternalism, giving people freedom to do what they want but setting things up so that the right choice is the easiest. Such evidence as we have, indicates that it works very well.

So should the meal plan serve alcohol, too, to avoid reacting to 'fears that they're "making a mistake"'?

That's a bit of a straw man. You can choose alcohol or no alcohol. But you don't choose mac n cheese or not -- you choose mac n cheese or some other food.

You can choose alcohol or some other beverage.

Choosing to offer only healthy foods at a location provided by the school does not preclude the students getting unhealthy food anywhere else. It's just making a healthy choice available, not making unhealthy choices unavailable.