Free throw violation -- why do we use the plane?

[bradfordwilkins]

Just refreshing up on free throw violations and the thought occurred to me... why does the foot crossing the plane come into play with this rule but almost no other violation (out of bounds, backcourt, etc).

Any idea on the logic?

Also, the rule doesn't mention any other body part which is what you see more of -- players leaning into their opponent or putting their arm out extended over the other players box.