Red Flush is regulated by kahnawake and powered by Microgaming casino games. Both kahnawake and Microgaming do not accept citizens from the United States of America.
I sent an email to the Red Flush support to get a clear explanation. Here is their answer :
Ronald here from the casino.
We currently cannot take new players from the USA at RedFlush. We do apologize for any inconvenience.
If you are a US citizen but you do not reside in the USA, you are more than welcome to register a casino account and play with us.
Please ensure that you do not attempt to access your casino account from within the USA as our systems will pick up where the account is being used and your casino account will be blocked immediately.
As part of the US gambling law, Red Flush preferred to step back from the US Markets. However, some online casinos still continue to accept US citizens. Check our top 3 US Casinos.
Technically, you could play to Red Flush if you signed up from abroad. For instance if you are in vacation in Cancun or in Panama, you could play to Red Flush while you drink your Tom Collins. However, you should not use a US address and an american credit card (from RBS for instance).
This is the same problem as "signing-up from abroad". You could use a VPN to play to Red Flush, but you would need to have a real address outside of the US (with phone bill at your name for instance). If you have your father abroad, for instance in the South Africa, it could work. Otherwise, in case you win a $5,000,000 jackpot, Red Flush could block your funds..
We know that Bovada is currently accepting US citizens. It is powered by Top Game software and offers a nice welcome bonus.