Volunteers volunteer their time and energy for free, so you are nice to them so they will continue to do so. The only part that matches your description is the

Volunteers volunteer their time and energy for free, so you are nice to them so they will continue to do so. The only part that matches your description is the free part and that part of it doesn't really matter because the point the sentence is making is "eventhough you pay them, treat them like they are doing you a favor for doing work for you".