Fluoridated toothpaste says not to swallow because it contains much higher concentrations of fluoride than drinking water. If you ingest too much toothpaste, it can cause your teeth to get blotchy: https://en.wikipedia.org/wiki/Dental_fluorosis
Yup, but even for fluoride gels it says explicitly not to swallow. One because fluoride is toxic when concentrated, two because it needs to be on your teeth, not inside your belly.
Once ingested, the fluoride has a systemic affect on teeth before they erupt, incorporating into the matrix of developing teeth to increase the mineralization content and decrease the solubility of enamel. [1]
