Why they use lens is because, Mister human need to warn Yukkuris by showing the cruel execution, since they invade human's territory on purpose. It's funny and bothersome way though.
Nature law will make the lens unbreakable if it means we can enjoy yukkuri's gruesome death full of suffering in detail Or maybe just break after everything end for a final stabbing.
Surely the Factory would also joyfully provide the lens for free if it's going to be used to torture yukkuri :)