Zonoma
Could it be possible that the phrase "speed when able" may have implied (rightly or wrongly) some discression? Maybe a more explicit clearance like "descend at Mach 0.8 then 280kts" might have helped nail it down?
I agree with 411A's meaning (though not his agressive style) that clearances should be complied with expeditiously (they are, after all, given for the best reasons!!) or else warning given immediately non-compliance is unavoidable, but very often communication problems can ocour in spite of relatively tight RT standards. I think that may be what happened here.