Er. That's a pretty big logical fallacy. Almost every substance ingested in huge quantities is lethal, including salt and caffeine.
If they're going to make you drink something daily for a lifetime without your informed consent, I expect some reason better than cutting down a few bucks of dental care expenses.
The fluoride is in enough concentration to be effective in the brief time it passes through your mouth (that's the idea, isn't it? Or is it supposed to somehow work systemically after ingested?). A tube of toothpaste will last a few weeks. There is, in principle, reason for concern. To claim it's scientifically proven to be safe you need lifetime double-blind studies controlling for any pernicious effects, which they obviously didn't run before starting this. A shame, considering there's apparently plenty of people who would be happy as uncompensated guinea pigs.
Or even better, by avoiding sugar and eating calcium rich vegetables.