When it comes to IRM protected Office document libraries in SharePoint 2013, Office Web Apps 2013 offers read-only capability. Office Web Apps relies on the document host system (i.e. SharePoint) to handle communication with Rights Management servers since it has no means to directly communicate with Rights Management Server.
IRM protection precludes Office Web Apps from allowing editing of IRM protected documents. Documents that are IRM protected through the client application (i.e. Word, PowerPoint, Excel) and stored on a document management system are not able to be opened. as well as IRM protected documents are stored on Windows Live, Facebook or other 3rd Party hosts systems.
Office Web Apps does not support the following features normally offered for non-IRM protected documents. These features are currently suppressed from the user interface:
- Edit in browser
- Copy selection
- Add comments
No word on when or if these features will be available for IRM protected libraries in an upcoming release.
Read more at source: http://blogs.technet.com/b/office_web_apps_server_2013_support_blog/archive/2014/02/21/irm-information-rights-management-features-in-office-web-apps-on-premise.aspx