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Based of my experience.
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).
This is just a generalization. Anyone can multi-account, no matter which background you have so long as you have an intent to cheat or game the system. They change it because they wanted to take advantage of an exploit that they found out, or they want to evade bans and restrictions. Whatever it is, the casino reserves the right to suspend accounts or to even ban them when necessary. They're a business to make money, not a challenge that needs to be broken by some individuals that found out some exploits and abuse it.