Since casinos are not giving any kind of free money and they have a house edge that guarantee the win for them in a long run no matter where do you bet or from which account, i.e. They aren’t either losers or winners. It’s usually used to prevent spamming by bots. The idea is that why would a bot use dual account while they can change seeds and do everything from one account?!
I’M afraid there is a something we don’t know about this If you ever visit scam accusations section, you should have seen atleast one problem from this kind from a very trusted known casinos which looks weird for me.
I think the most common reason a casino ban someone's account (because they have more than one account) is because he is trying to get the welcome bonus again. The welcome bonus can only be claimed once, so trying to get the bonus again is considered as cheating.
It could probably also be banned by trying to cheat the odds system with not allowed betting, but to be honest, I don't know exactly how it works.