I imagine that it’s because there’s likely to be a legitimate use-case scenario where you may want a background app to take screenshots. I’m not sure why that would be or what practical application there is for that, but this is probably meant to not directly interfere with background apps taking screenshots, but will still alert the user when it’s happening.
Apple already knows that iMessage, alone, is a huge selling point for their iPhones. They held out for a few years keeping iTunes away from the rest of us before finally giving in, but I very much doubt that they’re going to open up iMessage any time soon. It’s pretty much the only thing that keeps iPhone users in their ecosystem anymore.