[Jiocus]

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.

Use a straw, save a tooth.

[phonypc]

The pH of soft drinks is more typically in the range 2-4, due to the addition of various acids.

And the negative effect of soft drinks on dental health is primarily from the action of acid-producing bacteria metabolizing sugar. Diet drinks aren't nearly as bad for your teeth, carbonated water barely has any effect.

[Jiocus]

You are right.

Using a straw is helpful in all these cases.

[jack9]

Canned soda ph2.5 and is a solution

Fountain soda ph6.5 to ph8.5 and is a suspension

The differences are stark.