We have been seeing some issues with SharePoint IRM policies applied to a SharePoint document library where users are able to view documents with Office Web Apps 2013, but not edit them. This is expected behavior:
The issue is that when users attempt to preview the documents using Office Web Apps Server 2013, users will be presented with an error:
"Sorry, Word Web App can't display this embedded document because it's protected by Information Rights Management (IRM)"
As of this time, no further investigation is being conducted into this behavior.