...bouncing spirals off of the crossbar from the 50 yard line from his knees while shooting lightning out of his backside - you would be correct.
But to practice throwing actual passes that you would throw in a game, it's pretty helpful to be able to stand - and walk. Have you ever tried throwing with any kind of accuracy while balancing on one foot? Have you ever tried throwing with force without stepping into the throw?
You are right - an injured foot isn't the same as an injured hand/arm/shoulder when it comes to throwing. But lets not pretend that you can practice throwing with any kind of intensity while dealing with an injured foot.