Office Web Apps and IRM
In case you did not notice, Office Web Apps will not work for documents in Information Rights Management (IRM) protected libraries. If you try to open/edit a IRM protected document in browser, you will notice an error similar to this:
For IRM-protected document libraries or libraries containing IRM protected documents, it is recommended that the Default open behavior for browser enabled documents setting be configured to "Open in the client application" for a better user experience.