Based on the terms, US is one of the banned region so anyone live (temporary/permanently) in the banned regions will not be allowed to register. It is not an issue but it is exactly the rules, that's why you are unable to register.
I don't know Duelbits have changes in ToS.
Which casinos have IP blocks in place for restricted countries? has a
table shows USA is allowed, not actively banned.
The table was made 4 years ago and Duelbits team possibly made some changes in their ToS because there are many regulatory changes in the USA. recent years.
As far as I know it is something common to see many casinos that does not accept players from US, but there are still some casino that accept US players.
It is good for guests if they ban guests directly and don't allow account registration based on IP address. If they allow you to register account and only pull the term in ToS later when you submit withdrawal, it will be bigger problem with you.
Duelbits actually ban USA citizens.
Banned and Restricted Regions
Banned Regions
People living or having a permanent or temporary residence in any of these countries are not entitled to register an Account on duelbits.com or use any of the Service on duelbits.com:
Aruba, Austria, Bonaire, Curaçao, France, Netherlands, Australia, UK, Spain, Saba, Statia, St Martin, USA, or any other jurisdiction that the Central Government of Curaçao (formerly the Netherlands Antilles) deems online gambling illegal or are blacklisted.
Residents from the aforementioned territories are prohibited from opening an Account or in any way use or access the Gaming Services or any other Services offered by the Company.