They do at least periodically test whether the lifeboats will lower, but they do it when there are no passengers aboard, i.e. between a.m. debarkation and loading up the next group.
But, I agree that the passenger drill is merely a token nod toward fulfilling the law. I believe that ships sailing out of American waters have to do it before reaching international waters, as it is a US coast guard requirement. Hence the timing. And, even if folks aren't totally familiar with the ship's layout yet, the theory is that they should still know how to get to muster stations.
But, personally, I feel that most drills, whether fire drills on land or boat drills are pretty foolish. People will panic and while some may do better because of the drill, I think that most will not. It would be chaos.