The knee is actually composed of two joints, the patellofemoral and the tibiofemoral joint. The latter is generally considered to include two degrees of freedom; in addition to flexion/extension, this joint rotates in the sagittal plane (up to 70 degrees with the knee in 90 degrees of flexion).
Grood and Noyes(sp?) say this joint actually has 6 degrees of freedom, but I'm not buying it.
The patella also rotates on the femur as the knee flexes.
Thus, when done properly, the rotation caused by shikko and suwariwaza techniques probably don't overstress a healthy knee joint, because the knee is primarily in flexion during these techniques. Other pressures, I'll grant you, can cause injury in these joints.