Pending Transactions should be editable. For example, if I visit a restaurant and give a tip, then I cannot properly record this until after the transaction has cleared my bank/credit card which skews my accounting.
The reason why pending transactions downloaded from Online Banking cannot be edited is presented in the error message that you see when you try to do it - they can be changed. If you introduce some changes to a transaction where the category/payee/amount/date can change, it could lead to unexpected results.
We're constantly monitoring if we can improve handling those but at the moment there is nothing we can do.