Some people can be dubious and want to get more deposit bonus and register many accounts. If such person is known, the account can be banned. And in some cases that the user may not know about it and if he do not do anything dubious, the gambling site will merge both accounts into a single account.
I don’t even see the use of having multiple accounts because whatever game you want to play you can just use your first account and if your reason for having multiple accounts is for deposit bonus and offers given to new accounts i don’t think it’s worth it, why flood the casino with new accounts that you may end up not using? It should not be encouraged, if you need refer bonuses just refer a friend and keep referring people you know.