It has nothing to do with a user that deposits and withdraw even if have 100 accounts. In addition if he is a regular user, they should see activity on the other accounts as well. It could be a lost account and many more possibilities. Some users even say they don’t have connection with these accounts which extreme weird to me. - WTF the value of these accounts to cause the casino ban it? Did they earn million of dollars through spamming the bonuses ? -
That's because some users tend to use VPN while visiting the site and some sites probably have a tool where they'll auto flag users that have certain connections or link with another user. Spamming bonuses won't earn them millions but it's one way to make profits and reduce the house edge while playing. On top of that there are people willing to scrape every bonus they can find as long as they could profit from it. I know the rewards on most bonuses aren't that big but when you look at the perspective of the casinos it could cost them thousands of their profits.
In a casino that doesn't have any promotions like bustadice for example you're free to create several accounts and do anything you want.