I agree with Carl. I don't think this is any sort of conspiracy, but rather just poor education. The position that you are taking that CYA should not be used at all is just as extreme as a position that its level doesn't matter up to 100 ppm. CYA does have a beneficial purpose, but that purpose has been obscured by an industry that believes that full disclosure would reduce stabilized chlorine sales. So the problem is not that CYA is used at all, but rather that it is used improperly (i.e. not at an appropriate level and not with an understanding of the FC/CYA ratio roughly determining chlorine effectiveness).

Both the health departments and the industry correctly say that CYA protects chlorine from degradation from the UV rays of sunlight. Without any CYA in the water, half of the Free Chlorine (FC) would degrade every half hour in direct noontime sun. So some amount of CYA is needed to protect chlorine from such rapid breakdown.

The fact that is missing is that the reduced killing power of chlorine isn't just something like a 30% reduction or a factor of 2 or even 5 but rather a factor of 20 to 100 depending on the FC/CYA ratio. At an FC of 3.5 ppm and CYA of 30 ppm, 97% of the chlorine is bound to CYA and only 1.5% is hypochlorous acid (the other 1.5% is hypochlorite ion). Though that sounds bad, it really isn't because the other fact that isn't described is that it takes an incredibly small amount of chlorine to kill most pathogens. I wrote about this more in this thread.

So the right way to look at this is that you really only need an FC level (at a pH of 7.5) of around 0.002 ppm to kill most bacteria faster than they can reproduce -- that is, to prevent runaway growth. It takes a higher level of around 0.06 ppm or so to inhibit green algae growth (up to very high phosphate levels of perhaps around 3000-4000 ppb). To prevent person-to-person transmission of disease, the algae inhibition level of chlorine would be more appropriate for faster bacterial kill times such that 99% reductions in bacterial population would occur in a minute or two.

The problem is that one cannot maintain such low levels of FC since the absolute chlorine demand would quickly consume such small quantities. So some sort of chlorine buffer is required to provide a chlorine reserve while at the same time reducing its instantaneous effectiveness. If one does not use CYA at all, then the 1-2 ppm FC levels needed to not run out locally and to be more readily maintained would be at a much more powerful level of chlorine than is needed by a factor of 10-20 or more. This results in faster oxidation of swimsuits, skin and hair as well as faster creation of disinfection by-products and in some cases greater end quantities of such products. This is the typical situation found in most indoor pools because CYA is not used because only the sunlight protection effect of CYA is thought about and not its chlorine buffering effect.

Richard