Fluoride often appears naturally in water. Ground/Well Water in mountainous regions can often have 50X the fluoride content of urban water systems that are artificially fluoridated.
The idea that it is placed in water to pacify and subjugate the masses is a satirical comment that originated in Dr. Strangelove.
The only real argument that can be made is that it is a practice initiated for common good that violates individual rights. In other words, it is providing medicine against your will.