figure 4 toe holds and other twisting footlocks are allowed at brown and black belt level in some gi competitions - e.g. one I refereed at on the weekend. Heel hooks, neck cranks and a few other techniques are disallowed at all levels.You could make an argument about the fact that BJJ generally doesn't allow heel hooks, external figure-4 toe holds, and neck cranks, though. However, many BJJ guys also train and compete no-gi, which does allow those things.
Similar reasoning as to why attacks to the eyes and throat aren't allowed in striking competition.
Most BJJ students will learn how to apply such "forbidden" locks at some stage of their career, also how to avoid, counter or escape them.