It's allowed unless multiple accounts are used for "nefarious purposes". It's unclear what kind of purposes those are, but if a player isn't sure about it or has doubt if what he plans to do is acceptable behavior, he should have asked the casino. OP didn't do that. He assumed it was OK to open multiple accounts to bet above limits set on his older accounts.
While I regard the new account creation, liberal interpretation of rules and of the limits as willful negligence, using such vague terms in ToS as an umbrella to seize funds is simply too charitable.
I would consider the disregard of betting limits to be under the umbrella of "nefarious" if using the word to its extremes.
Perhaps a simple, "you are limited to $XXXX for sports bets," doesn't encapsulate Rollbit's intent enough. Instead, they should have written, "you, the person behind the computer and creator of this account are limited to $XXXX for sports bets, and should not create any new accounts to bypass this limit."