Thats the most ridiculous rule i've seen in golf. It's bad enough you can't ground your club in a bunker because you would be testing the sand, surely when you walk into the bunker your feet 'test' the sand anyway. But to be penalised two shots for hitting a leaf in your backswing in a bunker! Bit harsh if you ask me.
Don't think it's about testing the hazard as such but the fact that you can't touch or move a loose impediment in a hazard (rule 13.4c rather than 13.4a) which is what decision 13.4/33 refers to. The idea is that in a hazard you have to take your medicine. You can't pick up leaf or a pine cone or a twig that is behind your ball so it would be inconsistent if you could flick it away when making your backswing. Yes it's a tad harsh but that's what being in a hazard is all about.