Whoever is behind it as the person ultimately responsible must ensure that this does not happen, and if he lets it happen, he is allowing ban evading. And I don't care if the account is run as a commune or cooperative with no maximum responsible, they are as responsible as the individual person who is ban evading.
If a killer is working in a grocery store and that grocery store doesn't provide assassination service then that grocery store is not responsible for what its employee is doing outside the store.
When you represent some organisation but not as an owner, you are not representing yourself. So when you use some corporate belongings this doesn't make those belongings yours. Including corporate account. If that account wasn't use in personal purpose it doesn't matter who was behind the scene in any exact moment. Because it was an organisation not a person.
If you'll make an organisation responsible for investigating if their employee hasn't ever been banned on the forum they'll prefer not to work in here at all, because it is nonsense. Their account is representing them, not their employees, who can be changed multiple times during company work.