Wendy's by my house offers 99cents small frostys. If i go through the drive through, i will order a chocolate or vanilla one for the person behind me. They usually accept with a smile.
Keep extra change in your car, 99 cents to 2.00 thats typically how much a small frosty or shake is. local fast food places will like it because it helps promote their items. People dont think to order these little things, or sometimes they don't see it on the menu. Even if the person themselves dont like "sweets" they will give it to someone they have in the car with them or a child at home, or even the person behind THEM. Its allabout paying it forward