IndexedDB: calling IDBTransaction.objectStore() or IDBObjectStore.index() after transaction is finished?

Is there any particular reason why IDBTransaction.objectStore() and
IDBObjectStore.index() should be usable (i.e. return values vs. raise
exceptions) after the containing transaction has finished?

Changing the spec so that calling these methods after the containing
transaction has finished raises InvalidStateError (or
TransactionInactiveError) could simplify implementations.