Alaska was picked because North Korea was viewed ...
[link|http://www.defencejournal.com/2001/jan/shield.htm|US-NMD Programme-Will It Trigger A New Cold War?]
The NMD system is to be deployed in three phases. In the first phase, the capability -1 phase, which will become operational between 2003 and 2005, there are to be 20 to 100 ground based interceptors X-band radar based in Alaska which are primarily designed to handle, Small Unsophisticated threats involving approximately five warheads and only simple penetration aids. Alaska is the preferred site because it provided maximum coverage of all fifty US States against potential North Korean attacks.
But, you're right the earth is a globe... Recall though that later stages of the NMD system plan to have a site in North Dakota, presumably to better cover threats from the Middle East, etc.
Cheers,
Scott.