I think this is a misconception. It's not about the fountain vs canned or bottle soda.
Carbon dioxide lowers the pH of a drink to 4.5, which is acidic. The acid pH level erodes teeth and one shouldn't brush their teeth too soon after drinking soda because the enamel can be damaged. (Edit: acid on your teeth is always bad, brushing or not.)
Getting a soft drink from a fountain often means getting a straw for your cup as well. Using a straw to drink does help somewhat to bypass the soda from making contact with the teeth in the first place.
Carbon dioxide lowers the pH of a drink to 4.5, which is acidic. The acid pH level erodes teeth and one shouldn't brush their teeth too soon after drinking soda because the enamel can be damaged. (Edit: acid on your teeth is always bad, brushing or not.)
Getting a soft drink from a fountain often means getting a straw for your cup as well. Using a straw to drink does help somewhat to bypass the soda from making contact with the teeth in the first place.
Use a straw, save a tooth.