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)"
Microsoft is aware of this issue and is currently investigating. Updates will be posted to this blog when more information is available.