I would think this is an example of a pool store generalizing some advice that has a grain of truth without understanding why. When you shock a pool weekly using the pool store method, Cal-Hypo is problematic. Your CH rises and then you see a visible symptom with calcium flakes on the electrodes. The climbing pH in a SWCG pool can make a poorly maintained pool more prone to scaling when the CH gets high as well. Conclusion: Cal-Hypo is bad in a SWCG pool.
And they are right sometimes. Cal-hypo can cause problems in a pool, but so can chlorine, muriatic acid, CYA, baking soda, borax and washing soda.
Since most pool store workers don't understand the problems brought on by high CYA, they see trichlor and dichlor as better than cal-hypo because neither causes a visible side-effect that they associate with high CYA. They blame the algae and hazy water on TDS, phosphates or chlorine lock rather than the stabilized chlorine they are using for both routine chlorination and weekly shocking.
The problem isn't the stuff being added to the pool, but the fact that it is being added blindly without determining a need for it and without understanding everything it will do to the water.
That is the key difference between TFPC and the pool store method. TFPC relies on knowing your water and knowing the effects of adding chemicals, while pool stores (in general) rely on rules of thumb, generalizations and downright misunderstanding of water chemistry.