Here is Masters' scientific paper on SiFs (also called "fluosilicic acid" and "fluorosilicic acid").
In other words, even though neither the EPA or any other government agency has studied the effects of long-term ingestion of fluorosilicic acid, it is being used instead of sodium fluoride because it is cheaper.
(Na2SiF6) as shown in Figure 1.14 Although 25% of the utilities reported using NaF, this corresponds to only 9.2% of the U.S. population drinking fluoride-supplemented tap water. ... The cost savings in using fluorosilicates result in large systems using those additives instead.