The construction is similar. The buttons are metal and connect to a plastic "floating" part that in turns is what touches the real buttons on the phone. The glue is applied to the metal button to attach it to the plastic, no way it can touch the inside. And the plastic will stay floating.
If you disassemble a real one they are identical, in construction but the metal buttons are glued to the plastic. Probably with industrial CA. on the kockoff the metal buttons are just held by friction with no glue probably as a way to cut costs.
Umm, that's not the problem with the loosing buttons not for me at least..
The whole part that is floating is lost including the button.
The bumper i got from the apple store 3 weeks ago has plastic buttons which looks similar to metal buttons, i just had to try against the lid magnets of the macbook i have and they are certainly not metal!
So as far as i can see they are one piece togheter
That is if there isn't any earlier version of the bumpers that has metal buttons that are glued onto plastics (which doesn't really sound right, as it's not common to glue metal to plastic, which makes for a bad connection, but that would explain why the cheap ebay ones looses the buttons from the part)
EDIT: Found the button that i had lost recently and it's not metal either on the ebay bumper..