What – RMS protected documents cannot be viewed in Lync Sharing? 

Correct, and this is by design per Microsoft:

 

This is from the TechNet Article posted below:

You’ll notice some changes in the way IRM documents are handled by Lync in this release. Let’s talk about what we mean by IRM documents first.

Office Online defines IRM (Information Rights Management) as a means of “enforcing preferences concerning the transmission of personal or private information.” For organizations this also means ability to enforce corporate policy governing the control and dissemination of confidential or proprietary information.

When you open a document with such restrictions in an Office 2013 application, you’ll see a yellow information bar that tells you what you can and can’t do with it e.g. not allowed to print, forward, reply all. Also you may notice you can’t capture screenshots while the document is open.

On Windows 8, if you present such a document in a meeting by sharing your desktop or the program itself, you’ll notice that it will get blacked out. The thumbnails for these windows will also get blacked out in the present programs dialog to enforce the IRM policy. This way you won’t be accidentally sharing content that was meant to be for your eyes only. This behavior is not officially supported on Windows 7. (but I guess that it is happening)

http://blogs.technet.com/b/lync/archive/2012/11/02/presenting-your-screen-in-lync-2013.aspx