Surrounds are connected directly to the Beam via a private, one way 5 Ghz connection.  This is due to the low latency needed to sync them with the video.  If the Beam is powered off, that connection is severed. 

You would need to remove the Amp from the Beam in the setup prior to shutting off the Beam in order to use them as regular speakers, then add them back again to use as surrounds.

If you want to use the right Sonos lingo, when you say “zone” you mean “room”.

Looks like Sonos has revived Zone which they had dropped in the past and replaced with Room.

Unlike Shakespeare I suggest hanging the marketing department, then the lawyers. SIGH.

https://en.community.sonos.com/tutorials%2Dand%2Dhow%2Dto%2Ds%2D229149/sonos%2Dterminology%2D6905527#Zones

Thanks for the replies.  I should have mentioned these are all connected via ethernet  (Beam  switch   AMP) because setup required that, I assume due to the distance.  Mentioning in case it is important for this discussion.  

You would need to remove the Amp from the Beam in the setup prior to shutting off the Beam in order to use them as regular speakers, then add them back again to use as surrounds.

Yes, but this is a system setup hassle that does not pass the spouse test.  Rather than that, I would like the room to play the audio on whatever room speakers remain powered up rather than ignore the entire room.  Perhaps that’s not possible with the way Sonos software works, but wanted to throw this use case out there for consideration if there’s no way to do it today. 

It is indeed not possible.  As stated above, surrounds are directly connected to the main HT component (in your case, the Beam) by a one way private network as required for the low latency needed to sync them with video.  Because this private network is originated and managed by the main HT component, if you turn off the main HT component, the surrounds are no longer visible on the network.