@spivonious: They do that for any non-US citizen entering the US, not for US citizens. It's supposed to protect against terrorists somehow I suppose. I don't think it accomplishes much.
Of course, when I applied for my work visa for the US I also had to have my fingerprints taken, and then when I was called back to the counter five minutes later for my visa interview (which lasted all of 2 minutes) they verified the prints again. Just in case I had swapped places with my identical twin while inside the Consulate waiting room, I guess.