I guess Yamaha envisioned them to be used during performances, hence the name "Live" Control Knobs and in that perspective, storing their setting in registrations makes sense. My T5 only has 1 assignabe "knob" (slider, actually) and it works in exactly the same way, it even defaults to "volume" when the machine boots.
Not just these sliders, but also the pitch wheel, MOD wheel, switches, pedals etc. store their settings in registrations, but that can be overruled by freezing them when storing the registration. On my T5, the Assignable Slider can be frozen.
Regarding the memorizing: I make notes on the top of the song sheets regarding the settings of pedals, switches, MOD wheel etc. used in the registrations, so I don't really need to memorize them, but after playing each song often enough, my memory kicks in