Because it adds to the metal effect?It's metal. Definitely not plastic. Why would they put on the small, black plastic part to lower interference? Just wasting time and money.
It's metal.
Think about it. If it were really metal, wouldn't there be more left after blending it than a pile of dust and the metal bezel? Seems that the bezel was the only substantial piece of metal there. The back blended into a fine powder because it is, in fact... PLASTIC.
edit: here's another link, talking about engraving the original iPhone. Notice the part where they how how easy it was to engrave the SOFT PLASTIC?
edit 2: and another, more detailed link. 'Nice plastic that etches white-ish.' That's right, PLASTIC. Not metal.