I really don't think it's that sinister. I think the recommendations are a combination of two factors. First is that Dichlor-only use, especially when also using MPS as an oxidizer/shock, is net acidic so needs a somewhat higher TA level for a stable pH (50 ppm would probably be too low in this case and the pH may drop instead of rise). However, they are taking the pool recommendations and not accounting for the higher aeration in the spa, especially when jets are used. The 125-150 ppm TA isn't the common recommendation -- it's normally more like the 80-120 ppm range typically used for pools -- but of course you are looking at a specific recommendation that is clearly bad.
The other factor is that the people who really understand this stuff are deep in the bowels of some of the manufacturers and even then there may be some manufacturers that have no one that understands the TA/pH relationship that well. Many of these companies are really just reformulators or repackagers and not real innovators so they don't always have top-notch chemists who understand this stuff (not that it takes high-powered chemistry to understand this -- TA/pH is something one would expect undergraduate students to understand when looking at "systems" involving air/water exchange). I don't think it's intentional, but rather momentum of hearsay in the industry. By the time the information trickles down through manufacturer's reps and distributors to spa stores and eventually to the consumer, it's often misinterpreted or misused.
Anyway, thanks for the compliment. Regardless of the "why" for this poor information about keeping high TA level, I think we're in agreement that the correct information should be told -- namely, that if the pH tends to rise over time, then the TA level is too high because TA is a SOURCE of rising pH in its own right.
