The difference is intent. If a player has control of the ball and then gets down on the ground to head into the goal, then it might be silly, and some might make the argument for it being unsporting if it is just to show off how much time he has, but it is not a deliberate trick intended to circumvent the spirit or letter of the law. There is no benefit he is seeking to gain from the absurd action.
OK, I follow as has been said earlier - USB.
If this happens at the other end of the field, back to the keeper, I just don't see how that can be anything other than a deliberate trick to circumvent the law. You may disagree, but in my mind it meets exactly the same criteria as flicking the ball up in the air to head it. Not an illegal action anywhere else on the pitch, but clearly a player going out of his way to head the ball when the only advantage to be gained from doing so is the keeper being able to pick the ball up. In my opinion the same would apply if a player intentionally used his knee to nudge a ball that was on the ground.
Now, this is where I feel we need more clarification. To 'circumvent' means to 'go around' or 'get around' the laws of the game, which to my mind means, doing something outside the LOTG in order to avoid penalty or gain advantage. A defender, who heads the ball back to the goalkeeper has done what that is outside the LOTG?
Full back under pressure, plays a square ball across the pitch to center back in space, it bobbles a bit, but center back drops to his knees and heads it back to GK who picks it up.
How high off the ground does the ball have to be before he can 'legally' head it back to GK?
Is it our place to judge whether or not the defender has the ability to control a thigh high ball back to GK, or knee-high, or shin high?
As I have said earlier, if it is an obvious trick to eat time, juggling it up to his head, or GK tossing it for him to head back I could get them for time-wasting under USB, but if the ball is played to him and he heads it back to GK I would struggle to find an offence, however near the ground the ball is.
Again, not trying to agree or disagree here, I understand that 'circumventing the law' is punishable, I am trying to understand where the boundaries are between circumvention and quick thinking!