The Postal Office does the
same thing in Canada. You write to: Santa Claus, North Pole, H0H 0H0 Canada. There's about 11k volunteers, they reply in 22 languages and braille. Every letter with a return adress get a reply.
They talked about this once on TV and said every letter was read and they looked to see if any children confessed to being abused or anything like that. Apparently children often confess things to Santa, and they can often track down abusive families this way.
One thing I think that's neat is that if you use "track this letter" option it show it going all the way to the North Pole.