Pretty much this is why you should NOT blindly trust that omitting them is ok. If it should be there, SAY IT!
That they're optional in the first place was a pretty damned stupid idea... but that can be said about a LOT of the decisions made early-on in during JS creation that we struggle with to this day.
Whilst if you're looking for the "why" that's like asking why IE doesn't work like other browsers. It's IE, that's reason enough... in this case it's JavaScript, that's reason enough.