KD uses a system where nonprofit organizations provide volunteers to staff food locations in exchange for a charitable donation of a certain amount back to the organizations. CF then list the donation a Charitable deduction make it a even bigger win for their budgeting. All those folks you see in kaki aprons with temporary paper stick on name tags are volunteers not park employees.
From my experience, concession stands at various large permanent venues such as outdoor music venues and sports arenas have used this tactic for decades now.
It's not a new phenomenon, and though perhaps antiquated as opposed to other kinds of charitable fundraising activities the park could come up with, I don't think is necessarily an invalid option.
If this is a way for local or regional organizations to effectively fundraise in exchange for basic food service labor (also assuming all they're doing is serving and ringing up sales) during times of the year where they couldn't easily do the same at the more traditional venues, then why not as long as the volunteers are not being forced into it?