I do agree, I've seen countless inapropriate capes and I'm only using OptiFine, I never understood why capes weren't punishable.
However, there is one problem: you can give OptiFine capes to other people. This would cause a person not knowing why they have a cape and how to change the cape. A great example of this is the player "jeb_".
This player received an OptiFine cape but never changed the cape.
I know that it won't be very common, but it's still possible to receive a punishable cape and get banned just because someone else gave it to you.
Labymod capes are a bigger problem, you can just upload an image of something bad and boom everyone using Labymod (alot these days) can see it. If i was a 8 years old kid, i don't want to see a man without hat and alot of blood on someones cape. I think it has to be bannable. if the cape was gifted to you, you can always appeal and just tell the truth. ^^