It is if they add it to PUBLIC drinking water. You are assuming there is a benefit. There isn't.
I did a fair amount of research on this topic about a year ago. I had no opinion on the subject but was concerned about the dose my daughter was receiving. Her dentist was prescribing oral fluoride pills in addition to treatments and toothpaste. Her primary teeth were presenting evidence of fluorosis (which I later found the World Health Organization lists as indication of a toxic dose). What I found was that as long as the diet had adequate calcium and boron, there was no benfit to the use of fluoride for prevention of dental caries and increasing indication of neurological harm.
The final nail in the "pro-fluoride" argument came when I started searching the literature for indication of dietary fluoride deficiency in any field outside dentistry. Teeth are bone. If fluoride is a necessary constituent for proper bone formation, or augments healthy bone condition, one would expect to find that in the literature. I found nothing but the contrary.
By fluorosis, I presume you mean staining.
There are many parts of the country -- in Oklahoma, Arkansas, Indiana, Illinois, etc. -- where natural fluoridation of water supplies is sufficient to create staining.
To my knowledge, this has never been considered a health problem -- outside of cosmetics. Consequently, I question WHO's position on this matter and wonder if it might be alarmist, in the same vein as the enviro position on natural arsenic levels.
The physical effect of fluoride on teeth is to harden them. Now, recalling dimly from over 30 years ago: This hardening effect is confined almost totally to the surface layer. Because of this, the fluoride is best applied by contact, as in brushing or in topical treatments, rather than ingestion. Accordingly, I don't know why there would be any particular application for fluoride in bone formation.
Nor do I believe that fluoridated water is a particularly effective means of transmitting caries resistance. Good diet and dental hygiene can do the trick without any help from the water supply.