Unfortunately, it's not accurate to say there is no evidence. There
is evidence, it's just old evidence. For some reason, Newsweek decided to focus on the fact that there is no evidence that fluoridation is helpful for
adults, and that the evidence for children is almost all from before 1975. That is important since behaviors may have changed post-1975.
The "Plain language summary" from the authors - their words, so this is the authors of the study synthesizing their conclusions for a lay audience - is clear:
Data suggest that the introduction of water fluoridation resulted in a 35% reduction in decayed, missing or filled baby teeth and a 26% reduction in decayed, missing or filled permanent teeth. It also increased the percentage of children with no decay by 15%. Although these results indicate that water fluoridation is effective at reducing levels of tooth decay in children's baby and permanent teeth, the applicability of the results to current lifestyles is unclear because the majority of the studies were conducted before fluoride toothpastes and the other preventative meaures were widely used in many communities around the world.
There was insufficient information available to find out whether the introduction of a water fluoridation programme changed existing differences in tooth decay across socioeconomic groups.
There was insufficient information available to understand the effect of stopping water fluoridation programmes on tooth decay.
No studies met the review’s inclusion criteria that investigated the effectiveness of water fluoridation for preventing tooth decay in adults, rather than children.
This is from http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD010856.... Thanks to HN user giarc (https://news.ycombinator.com/item?id=9807427) for pointing it out.