And that's where it's not clear. How would they are going to define nefarious purposes.
Well, that's the thing. Anyway they want to. It's up to them to decide if what the player does is nefarious. Accepting the terms means agreeing to all of them. And one of the points is that they can close an account
for any reason they want. It's bullshit, it's wrong, but hey, gamblers decided that traditional fiat casinos weren't good enough. In that case, enjoy the "safe wild wild west" of crypto gambling. Since OP's account got locked, it's safe to assume that his use of multiple accounts was considered as nefarious.
If all looks correct then why would a user will create more than one account at all?
There aren't many valid reasons to be honest. One I can think of is if you lose access to your 1st account or you get it hacked and you can no longer gamble from it. If the associated email is gone as well, you will have no way to recover it. Maybe the support can help, but who knows? Creating a new account in this case would be a justified reason.
The arguments can be made from either side. Honestly, having a rule allowing multi account with a very unspecified term does not look good for a gambling website. It must be either you do or do not. Gambling sites are always hiding their bad practice behind their shady terms and for crypto business it more easier to abuse users because there are very lower chance to take legal actions by the customers.