Both LAX and ANC air traffic give clearances for departing airplanes to exceed 250K by using the expression "Cleared high speed climb". If they want you to maintain 250 they will say "Maintain normal speed". I don't know what authority they have to do that, but they do it anyway. If you question them they tell you to use "pilot discretion".
I have noticed though that many pilots do not understand the difference between the calls.