As far as I know having multiple accounts from one owner is prohibited by every online casino there is because of people taking advantages of their bonuses and promotions.
As long as all rules are respected a person can have as many accounts as he is want. As you said people that have multiple accounts usually take advantage of bonuses and that's when their accounts get blocked and they come here asking why. But as long as you respect all the rules and don't abuse promotions you can use multiple accounts.