So if someone balances on one foot for an hour without putting down their other, and then goes towards the basket, that is a foot fault? Not crossing the mark until the disc has settled is an easy way to determine balance. It is a set amount of time to establish balance and takes away the guesswork. What other logical marker for the correct amount of balance time is there?
But the exact same footwork with a longer/higher/slower putt should be a foot fault when it wasn't before? Why should how many times the discs wobbles before resting have anything to do with the legality of your putt?