This is what I thought also. Silly rule....you can be facing someone who is swinging it a mile, if they hit you on the full in line the umpire must assume it is going straight, not taking into account any swing/movement.
Imagine a right arm leg spinner bowling over the wicket. Turning it a country mile. Bowls a full toss and hits the batsman in front. Umpire has to assume that the actual path of the ball would have continued. He can't make the assumption that the the ball would have turned. For the swing bowler, he needs to take into account the anticipated path of the ball.
In assessing whether the ball would have gone on to hit the striker’s wicket, it has to be assumed under Law 36.2(b) that the path of the ball before interception would have continued after interception, irrespective of whether the ball might have pitched subsequently or not.
In the case of the leg spinner above, chances are the batsman will be given out.
The left arm bowler bowling round the wicket, the only consideration the umpire needs to give is of the expected path of the ball. Just because the ball hit in line with leg does not mean that you should have been given out. The assumption here is that the ball has somehow straightened from its original path, if the umpire has assumed this, he is wrong.