I firmly believe that the most important physical aspect of shooting is the RELEASE (the moment the ball leaves the fingers will either make or break the shot).
Therefore, if you already have enough power to get the shot up, why add in a heavy knee bend (which is an additional variable that must be controlled)?
Many of the best free throw shooters (Steph, Ray Allen, Jamal Crawford) have a very minor knee bend and recruit minimal power from their legs- while almost all of the WORST free throw shooters (Jordan, Thompson, Drummond) bend VERY low, even by a guards standards.
Efficient shooting is all about ELIMINATING excess motion and MINIMIZING variables! It's no wonder these big men always shoot the ball long. I believe that if more big men would stop trying to shoot textbook shots (according to 20th century standards), and NOT bend their knees heavily, they would see major improvements.
I haven't attempted a free throw streak since I was 15 years old and made 119 in a row. Even at a young age, and being very weak, I didn't bend my knees on ONE of those shots- and I still don't bend on free throws. The fewer variables you have to control and execute to perfection, the simpler your shot will become- and the more you'll make effortlessly.
Note: I'm not telling any of you guards to not bend your legs. I'm talking to 7 footers and elite shooters only!