Fees and categories, when I designed the Finance modules, seemed like the kind of information you would not want to delete, as they constitute a form of financial record. Rather, you can set them to be inactive, so they show in the management view, but not when setting up invoices.
If you believe fees are deletable, you can always do it through the database directly. In the future we could look at given the option to delete fees, but right now we have our hands full with other things.