Multiple-account are most the time from "Sportbook" user. Why? even they're not taking any bonus, they trying to multiple-account the casino because new account is always have limited amount to be betting for sportsbook.
That's why, they multi-account on the site. Sometimes, there has some unwritten rules in casino (Multi-account can be acceptable as long the user not using anymore the old account or trying to abused promotion or referral).
It is true on some casinos. They allow you to create your second or third account and use it with a condition that you abandon your previous account.
I would like to go first to ask support team about my previous accounts if I forgot about those ones. I don't want to have issues because I don't go first.
I read some fake scam accusations of cheaters tried to attack casino. Those cases end like they have multiple accounts, and have similar betting pattern in a same tournament, same type of sports, same ways of betting. It's very hard to see a trusted casino handles those cases against users not correctly. They know that incorrect handles are harmful for their reputation and they are always careful about their investigation and decision making.