Uh, the adapter has a ground contact, it's just you have the option of connection said adapter to a three pin (grounded) cable *or* hooking a two-pin folding plug directly on, which makes it nice for travel! (To put it another way, you can convert an Apple AC adapter from a typical tabletop unit to a wall plug pack.)
And pretty much every outlet in the US is grounded, the exception being really, really old houses, but they've become the exception and not the rule in the last 20 years.
Sent from my Tablet