Part of your title is cut off, I assume you're referring to the "where VS1 is determined at the maximum takeoff weight" part.
Basically, it's a requirement that VMC not be so high that it drives the takeoff speed schedule at all weights. You're allowed to have a VMC which is high and limits V2, but not EVERYWHERE.
Part 25 is similar IIRC.
There's no positive rate of climb requirement for VMC because there are other requirements which deal with having the ability to climb at takeoff speed schedules; this is just a controllability requirement which provides one of many constraints on the speed schedules.