The distinction relates to the traffic environment.
In the US there may be VFR traffic in the 'CAS' system, working on 'see and avoid'. Therefore, ATC cannot relax the 250kts speed restriction.
In the UK, Class A airspace is an environment in which all traffic is (a) known and (b) controlled, so 'see and avoid' is unnecessary (though it remains the final means of avoiding collisions, as the UKAB constantly reminds us), therefore, the speed restriction may be relaxed by ATC.
The pilot is at all times responsible for navigating the aircraft and for complying with SID tracks, though he is not provided with the relevant information to do this (in sufficient detail).
The pilot does, in the circumstances in which 'No ATC speed restriction' has been communicated, retain the right to fly at standard speeds. We should bear in mind that the ANO says, basically, that the speed restriction is 250kts below FL100. If a policeman told you 'you may drive along the motorway at 100mph' would you do it?
Hope this helps. It's worth trying to hang onto the 'big picture' before reaching for the books.