As @TellowKrinkle said, it’s to save bandwidth on those responses and make them easy to parse. Delivering images via JSON is super duper ugly, as Base64 encoding is the only real way to do it. The encoding process increases the image file size, only adding to the bandwidth problem.
BUT! We know that the API v1 image/custom font/Unicode setup isn’t the cleanest. You should should check out API v2 — the way the images are delivered is way better (we think), far more consistent, plus you get access to SVG versions of all the radicals. We give the content types and the full URL to the image in there, not just the file name, so it’s easier to link to it or grab it.
Let me know if there’s anything we can help out with. 