If you remove this restriction, you add another tool for users that want to troll the site. Being able to post NSFW images of all kinds could cause a lot of trouble. There are all kinds of images floating around the internet that you regret ever looking at, I think it could be a bit too dangerous to allow any user to plaster those across the site.
This is a point where you have to decide between the potential harm done by the restriction and the potential benefit. How harmful this restriction is depends a lot on the specific site, on SO posting an image is rather uncommon, most questions consist of some text and a code block. On sites like Photo or DIY posting an image is very common, so the restriction should be disabled there (and it is, I think).
What we should do is to make it clearer to new users that they can't add images and warn them earlier, the current UI is rather bad and misleading in that aspect.