As already said but, it's that way because the rules make it so. It's kind of like what your Mother used to tell you "because I said so." F1 can't fake to 1st while in contact with the rubber, period. F1 CAN fake to third in contact with the rubber, period (provided he steps in the direction of the base).
Has to stop...he's deceiving the runner? Isn't that what a pick off is supposed to do?