I know the studies were not done because because a news article mentioned that this study was done precisely because only a few flawed studies were previously done.
Pregnancy is a common condition. Any product must be tested for against reasonable exposures. That isn’t a “silly belief”. It is due diligence. Especially when alternative delivery mechanisms, which would not expose potentially vulnerable segments of society such as pregnant women, are certainly possible. Fluoride could be delivered via delivery systems that would not expose pregnant women. By electing to use a general public means for delivering the fluoride, those who made that choice have the onus to make sure it is safe.
This doesn’t even consider groups who could derive no benefits whatsoever from fluoride but still be exposed to potential harm and other groups who, although they might have fewer cavities, face grave consequences being exposed to fluoride that far, far out way the benefits.