He can't. It has to pass for him to be a able to veto it. Currently it has only passed the house. It has to pass the Senate. Then it will come up on his desk for his signature, and then he can veto it by refusing to sign it.
Him saying he will veto it, is like a courtesy notice to the Senate saying "don't even bother with this, it will get vetoed."
NDAA is a matter of international terrorism. It also heavily depends on how well the federal government does its job, no matter how distrusted the feds are. There is no sense in innocent individuals being locked, and even if it is, it will almost never happen.
CISPA won't kill anyone, it's a matter of top-notch economy for a top-notch sector. The choice is much more easy here.
