Yes, it is for the M203. I'm guessing the only reason manufacturers put those on AR15s is because people want to buy a gun just like what's in the military. I'd say it's the same reason as why the bayonet lugs are on them.
Very true, but it's a shame so many ARs are built to cosmetically look military but otherwise costs are slashed any way possible to avoid adhering to military specifications. It's not so much an issue in a target or varmint rifle but one that's sold as an "M4gery" and has the M4 barrel cutout and bayonet lug, well why not do as much as possible to make that model milspec.
It really doesn't cost that much more to use more expensive materials and specs. PSA does it and sells in the price range of the corner cutter ARs.