Perhaps they needed to understand enough magic to survive such problems, but that doesn't mean they needed to devote their lives to it or follow the ways of the magi to the letter, as Rin proves conclusively. Yet, he never gave Sakura that choice, and he didn't really give Rin much of a choice either.
He essentially told Rin that, not only must she learn magic, but she must live the life of a magus, and he put Sakura in a situation where she could do nothing but live the life of a magus, and where she would not even have control over that, thanks to Zouken. Further, he compounded it by forcing them to avoid each other and forcing Rin and Aoi to act as if Sakura did not even exist.