Turns out there is a slight bug that seems to prevent you from doing interactions (update, scroll) with the results of a JPA named query that you dropped on a page using ADF Binding.
People are running into this when they are doing the EJB tutorial on OTN for example.
The problem is that the way the binding is set up for you automatically doesn't allow you to actually access the iterator set of records to do follow up operations.
When I last checked this was solved in the next release of JDeveloper, but in the meantime there is a quick simple way to resolve the issue by changing the refresh condition of the oiterator in your page binding.
Here is a little demo that shows the problem and the solution: