They acknowledge at the site this one seems iffy... I could understand a screen or a speaker covering being punctured.

It seems that it isn't the thick part of the strap that is breaking, it is the string bit that attaches to a Wii.

I agree that it is highly unlikely to happen but a small chance times a large number of uses = a number of times it can happen.

So sheer large number of consoles sold and a few incidents will happen.

Universal rule of product design:
No matter how foolproof the product is, nature will breed a 'better' fool.