There is absolutly nothing inherent in tai chi that should create knee problems. I think a better anatomical example is knee over foot. This describes both important allignments. The reasons for not bringing your leg back past 90 degrees is a principle that most people have recieved instruction in at some point in their lives and is very visual. At our school and when teaching outside classes our instructors use a system we call Student Assisted teaching.
Students pair up on many movements for the purpose of giving feed back.
This gives both partners imediate feed back and creates some muscle memory.