Cap and helmet - are the Laws inconsistent?

In the 20/Twenty final the following happened:

Batsman hits the ball into the deep; fielder running round to field the ball slides to stop a boundary; his cap accidentally comes off; the ball is deflected by the fielder and hits his discarded cap. Batting side ask umpire for five penalty runs, umpire says not a wilful act and no penalty runs awarded.
Commentary team debate this and Mike Atherton reads from Tom Smith's New Cricket Umpiring and Scoring stating the umpire made the correct decision, then one of the commentators refers to a ball hitting the helmet placed behind the keeper, claiming that this is not a wilful act, yet you get five penalty runs for it.

What is the difference? If they are right, isn't the Law inconsistent?

They are both right and there is no inconsistency in the Laws.

Had the fielder deliberately removed his cap and left it on the ground, but without actually using it to field the ball, the ball's coming into contact with it would be classed as illegal fielding - it was a wilful act that put the cap there - and 5 penalty runs would be awarded to the batting side. Had the fielder used the cap to field the ball, then, in addition to the award of penalty runs, the incident would be reported and appropriate action taken by the Governing Body.

The helmet has been deliberately removed and placed on the ground, so it presents the same situation as the cap deliberately removed; it is on the ground as the result of a wilful act. In the match in question, the cap was not removed deliberately, it simply fell off, so the ball's coming into contact with it was not the result of any action by the fielder. Under such circumstances it would be most unfair to punish the fielding side with penalty runs - and the Law is not unfair!

The following table sums it up

Ball coming into contact with	Penalty runs	Report
Cap fallen off	No	No
Cap deliberately discarded	Yes	No
Cap deliberately used to field ball	Yes	Yes

Read more about Law 41 (The fielder) at the MCC website