IP multicast (224.x.x.x) addresses have been hardwired to a range of Ethernet multicast addresses (01:5e:00:xx:xx:xx) for a very long time, and unfortunately overriding that mapping doesn't seem like it would accomplish anything, because any Ethernet switch is just going to broadcast anything it doesn't recognize anyway.
A practical example of this is where you have multicast IPTV traffic arriving from a ISP network into a home/office network. You generally have to filter that out and direct it over a separate network of some kind to every IPTV device, because if you just forward it directly onto the local subnet, inexpensive switches will broadcast everything to every port, which is a problem.
Does anyone really want to run a separate network to their set top boxes simply because link layer multicast is synonymous with link layer broadcast? It makes it difficult to watch television on ordinary desktops because they are connected to the wrong network, for example. Perhaps inexpensive Ethernet switches will implement IGMP snooping in the future for this reason. It isn't common yet though.