From my interview at facebook some four years ago, I am inclined to believe they disable intelisense so the candidates do not get tangled up in the minutiae of language syntax, function arguments, etc. and instead focus on the algorithm they are trying to write.
Except you get dinged on your performance if your code doesn't run once the interviewer has a chance to test it after the interview. And getting dinged generally means rejection in today's economy.
