GrandParker is regulated by Curacao and powered by RTG casino games. Both Curacao and RTG do not accept citizens from the United States of America.
I sent an email to the GrandParker support to get a clear explanation. Here is their answer :
GrandParker is an excellent US Casino. It is my best.
As part of the US gambling law, GrandParker 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 GrandParker if you signed up from abroad. For instance if you are in vacation in Panama or in Colombia, you could play to GrandParker while you drink your Pina Colada. However, you should not use a US address and an american credit card (from Chase for instance).
This is the same problem as "signing-up from abroad". You could use a VPN to play to GrandParker, but you would need to have a real address outside of the US (with rent receipt at your name for instance). If you have your mother abroad, for instance in the Sweden, it could work. Otherwise, in case you win a $6,000,000 jackpot, GrandParker 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.