I claim no expertise with the FARs but have had involvement with the equivalents in other states. Reading the regs that you link to, it seems very clear to me that ATC is inviting you to get a slap on the wrist (or worse) if you exceed 250kts before passing 10,000ft. Para (b) of the regs is an example of the structure used when an exception can be authorised/instructed by ATC.