I do not see any good reason for them blocking multiple accounts in casino gambling by the casino platform because it has no negative effect to the platform. As long as those accounts do not go against the rules of the casino platform. They need more users and that is also part of getting new users into the system. The reason for banning multiple account in the Casino platform should be that they either violate the platform rules or cheat other users int he platform.
I don't know why they don't allow multiple accounts but I know that it is not a problem to them because you did not harm anyone. When it will be a problem is when you it is in their best ToS and you violate the law. I don't think that they will do anything until you win big because they are looking for every means not to pay you your wins.
I believe that is why they put that in their ToS so that you don't blame them for not paying your wins. I believe they want to make sure that the total number of accounts that is on their platform should be the same number of gamblers that use their casino so that they can have more money.