I would not say this is an issue. The host account is a account to admininistrate the DNN System on a higher Level e.g. adding new sites to the Portal, that is completly different to existing sites.  This means if you run a DNN Hosting with DNN the systemadministrator should not be able to delete any Content for security purpose.  It is a Supervisor account.

So what should be the solution then?

You can add the host user (users) to the Forums with the administartor permissions.  This should then be work.