First off: Green Day > Guns n Roses. I don't care what anyone says.
Second: Who did people expect them to choose? They always go for popular artists who have name recognition. In fact i'm surprised they didn't do the obvious and pick a band like Avenged Sevenfold or something. Hell, for all we know Green Day could have been influenced quite a bit by GnR.
12/27 - Protest the Hero
02/05 - Machine Head