Thank you for writing. I agree that this is not consistent with the standard WinForms model but I am afraid that this is a legacy API that we have to support in order not to break any existing application. System.Object can not be directly added as an item to the RadListBox and you will have to create a RadListBoxItems and decorate them with the necessary data manually or use data binding. Please write again if you have other questions.
All the best,
the Telerik team