The question isn't whether it's creative or slippery -- is it within the rules?
This is an exception based system -- so what is the general rule, what is the exception. The exception overrides the rule.
My analysis would go: General rule is that attacks go against creatures. The exception allows for attacks on a creature or an object.
Is "the ground" an object -- not within a strict definition, but a blade of grass, or a rock, or a coblestone are objects.
What justification is there to deny the attack on the object?
The player gives up a chance at high damage for an improved chance at lower damage.
Where is the rule which denies that interpretation? Why would we want to? Is it harmful to the dm if a player wastes an encounter power on minions? Is it difficult to compensate by spreading minions out?
- - - - -
I think the effect is so situational that it makes sense to allow it. Certainly if they targeted a chair or a pillar you would agree that this is "an object" why can't a flagstone or a floorboard be one as well?