It isn't the strap that isn't sturdy, it's where it's connected to the controller that breaks, and while it may be a disign flaw on Nintendo's part, it isn't their fault that it breaks at a certain velocity, because they didn't design it to be playable at any relatively ridiculous speed. It was designed to be played a certain way, and since we don't have a majority of Wii controllers doing this, then that means that people must be doing something with it that isn't normal. If a lot of controllers started doing this, then Nintendo might be liable, but a few instances like this, no.
And if you want Nintendo to be responsible when it slips out of someones hands, that's pretty ridiculous.