The US doesn't conduct a nuclear first strike, not because it fears getting nuked in return, but because it doesn't want to be seen as the bad guy. The presence or absence of a missile defense system doesn't change this. Also, there's an entire paragraph on why even an effective system won't change this, so long as it's not perfectly effective (which it can't be), because we still end up with one or more nukes landing in the US, and the acceptable number for that is politically 0.