Ah, okay. You don’t know how colours work. That explains a lot.
If the red channel is 0, the saturation is 0. Which means the texture is fully desaturated. If the red channel is 1, the saturation is whatever the saturation of the sampled pixel of the texture is. It’s clamped to 1, because making saturation greater than one will break the colour.
“Hey system, re-adjust everything so that the highest value you got is always 1 and everything else is scaled down appropriately”.
This would result in a saturation adjustment that does next to nothing. The whole point of adjusting the saturation of a texture is to adjust the saturation, not “keep relative saturation levels of all the pixels exactly the same.”