W777-
A couple of our pilots reported the same "clearance" out of LAX and DTW a few weeks ago. We researched it fully, and the controller has NO authority to waive the 250 knot restriction. As Wino says, IAH is the only airport in the US that allows it on departures.
If you're within 12 miles of the shoreline, the max speed is 250 below 10,000. (Unless your a/c requires higher.)