Why did Sherlock fake his death this way? [migrated]

In S2E3 Sherlock faked his death in such a way that the only person who have seen it (from the right angle) was Dr. Watson. Other people present on the scene were actors (Molly and homeless people).

If I understand correctly, Moriarty had a sniper watching whether Sherlock will jump and that will act if he does not. But given the explanation of how Sherlock faked his death, it is obvious that the sniper would have seen that it was faked. So why did he trick Watson instead of the sniper?