This is true, we can't and won't waive the 250 knots below 10,000 limit. The reason these instructions sound like that is most likely that ATC anticipate not being able to get back to you and issue the speed when you are out of 10,000 and want to catch you before you left 10,000.
Now, I do want to mention that the clearance may sound confusing and the controller issuing it should have foreseen it. The correct way by our bible, 7110.65, would be 'Leaving 10,000, increase speed and maintain 280 knots' that way there is not a single reason to question it
