Actually, this is expected behavior as this is how masterpage is used. The masterpage is used for building the layout that will be used for multiple ASPX pages. The ContentPlaceHolder is not an iframe, but an editable area for the ASPX pages.
In order to achieve your goal you can use our RadSplitter control with two panes - in the left pane you can place the panelbar and the new paheges will be loaded in the other pane. Please take a look at this example
that shows the needed approach.
the Telerik team