Dicecoin.io redirects to duckdice.io for certain when I go there. It's because I'm going to it from a US IP and if I use a VPN I can actually get to dicecoin.io. Cryptogames.io also blocks USA users although that was a later in the history of the site when they started doing that.
USA is blocked by many dice sites but are you sure dicecoin redirects to duckdice if we visit from US IP? This sound quite serious now, if all three of this sites are connected than duckdice might also loss all the reputation they have earned so far. I have also played there and really like the user interface and support duckdice have. 
Yes. I'm 100% it redirects to duckdice. I have ever actually seen the dicecoin.io page until you mentioned it not redirecting for you.
Tested (from usa):
It's a referral link, but not sure why would they redirect to them. It's indeed a possibility that cryptogames.io = duckdice.io = dicecoin.io
if it is a referral link,then it makes no real sense
also just tested it from my country (not US) dicecoin loads as it is supposed to
so this redirect is most likely to cut the US customers,many sites are doing this to avoid legal actions or problems in the future
imho there is no connection between dicecoin and duckdice,looks like totally different concept and developpers were not the same
Well it makes sense that it's a referral link. Probably the owners of cryptogames still want to earn something if the users from the USA and uses duckdice for it.
Would there be also that much rumour when they used Primedice for it?? They just picked a new dicesite because the chance is bigger that the user doesn't got an account there.