I don't know what the origin was but most applications of the 250kt speed limit appear to be in airspace within which VFR flights might be operating (presumably to give them a fighting chance to see and avoid).
In the UK the limit only applies in certain airspaces/situations - which would seem to support this theory.
Returning to the orginal post, the 'no ATC speed restriction' may well be deresticting a procedural speed limit that is specified as, say, part of a SID or STAR. Certainly in the UK, individual controllers do not have the authority to permit aircraft to break the law.