These are being used as link addresses, they probably aren't even routable within Amazon's network. They probably want the link addresses to be globally unique so it simplifies management or so traceroute reverse lookup is easier (e.g. to pinpoint traffic loss). They probably ran out of the other RFC3330 usual suspects or something.

I wouldn't read too much into it. If they are using it in a way that will break if 240/8 is assigned, I'm sure they will fix it quickly.