tired,
You're 100% right. But, to continue the example above (and this has happened to me more than once usually in France or Spain), what do you make of the next instruction?
ATC 'Blockhead 123, report your speed to XXX on 123.45'.
This illustrates that the controller does expect the aircraft to fly the speed. In the first exchange, had Blockhead 123 concluded the conversation by saying,
'Blockhead 123, confirm we may vary our speed?',
or something similar, the problem does not arise. It's slack R/T on the part of the controller, but the crew should ealise this and take steps to clarify their clearance.