The purpose of this optional requirement is to make plastic recyclable. Remove the metal bits so the plastic can be shredded into granules that can be reused.
It appears to be intended to avoid the requirement of a post shredder metal separator for recycling. Metal is separated out by vibrating, magnets and sifting. Meeting this optional requirement would presumably reduce the cost of recycling, at least in the face of third world labor costs versus first world recycling machinery. The economics of e-waste export are predicated on deadhead cargo capacity and lower manual labor costs. The latter of the two unsustainable over the long term and exploitative.
Use of machines to separate metal is part of 'single stream recycling' which is predicated on the impossibility of providing custom tailored recycling methodologies for particular articles or products, which could number in the millions. Simply grind it all up and separate it afterward. And of course there are exceptional handling cases like CRT based monitors and TVs. Just not metal threaded inserts in plastic cases. Labor costs and volume have driven household waste to become single stream recycling, lacking the option like e-waste to export garbage to the third world for most locales.
Here in New Zealand we have a lot of people fond of insisting on strict adherence to rules ('alles in ordnung' with fascist overtones). Rules for rules sake. This particular issue is terribly misstated and inapplicable. Separating other material from aluminum (alloy) is the issue for Apple's cases. Do we have a need for more requirements?