I have seen many programs using this.externals, but only for basic things like accessing the HTML5 canvas or other small hacks. I've never seen a program use it for evil purposes or anything that could be harmful. This has led me to wonder why this.externals is bad enough to be against the rules.
I once heard that it can be used to load images that are adult rated, but I've never seen such a program attempt this. People could easily bypass this with Bob's Imageinator anyway, or by using shapes to draw something adult rated. My point is that banning this.externals doesn't seem to do anything about adult content. I'm not saying it should, I'm saying that it must be a different reason.
My better guess is that it can be used to load copyrighted images. If a user published a program loading a copyrighted image, Khan Academy would be responsible for it. This does make more sense because Khan Academy has no issue with images on Wikipedia - they are all free to use.
If it is a different reason altogether, feel free to provide corrections.