In most countries ATC can allow speeds higher than posted, but in the US the 250 or minimum safe speed limit cannot be relaxed by ATC except as shown in Notams. It is a safety requirement based on the large number of light, VFR traffic.
LAX gives "High speed/Normal speed" because many pilots operating into there are not aware of FAR91.117 which allows speeds above 250 if required for safety of the flight (such as a heavy 747 on departure). They are sick of wasting time telling those pilots what they should already know, and so will issue a 'high speed' clearance to every airplane on departure, even if you don't ask. It does not mean unrestricted speed, you still have to comply with FAR91.117. Of course, it is not necessary to ask for min safe speed on departure, but those pilots trained in Europe and Asia will continue to do so, since they are not aware that rules are different in the US (an ICAO country, nevermind).
In SFO (and other places) you will be given a 250 speed restriction on the SID. This is for ATC speed control and in that case you will have to request a higher speed if you need it for minumum safe (flaps up).
I will take the opportunity to plead with you all who fly into LAX to keep the readbacks to a minimum. It is not Europe and the controller is not necessarily listening to you when you read back every item on the transmission recieved; he is often listening on two or more frequencies and is just waiting for you to shut up. The rest of us are waiting for a chance to get a word in. This is especially true on Ground freq. The ATIS tells us to only readback 'Hold Short' instructions. it would help a lot.