Honestly, I'm not so sure. Looking at the APIs exposed by Facebook you'd be hard pressed to find anything that looks like its engineered. It seems to be a hodge podge of features thrown together over years with the stuff that works best/well enough sticking around long enough to be seen by the public. I think its a safe bet that many of the things not exposed are quite messy/nonsensical.

They could very well have never thought of deleting a photo; it doesn't seem that there is much interest in information removal there, and I could easily see if being a very ugly manual process that no one wants to spend time doing and as such it never gets done.