If entire cl fails, why is professor fired?

It maybe that the students REALLY did fail the cl and did not put in the required effort. A south korean professor was fired from my university, because nobody got higher than D grade in the cl. He was calculus professor and he held phd in mathematics. He said he cannot p students if they don't meet the course requirements. He refused to curve or give a grade boost to anyone. Later some student claimed that he was racist and so he grades too harshly. However, i took him as statistics professor and he gave me B grade. He also wrote me recommendation letter for graduate school of math. I think he was good professor and very fair. I do feel the whole cl really did not p because they lacked the will to study or make effort.