The "good reason" you can't use undersized cables is because the OCP won't protect it in case of a fault. If there was a fault, the cable could burn up before the OCP trips.
I suppose you know that puts you at odds with 240.21(B).
As long as the tap is protected at the load end from overload they believe the larger line side breaker will protect you from overcurrent (short circuit) faults in the tap.
That is why there are different rules for 10 foot and 25 foot taps.
