Note that the behaviour is that playback starts on *both* the client and the server, it does not just play on the server. The zone in which it plays back in the server is the zone selected by the server zoneswitch rules.
re jremote, I'm really not sure what the intended jremote behaviour is, I tested again yesterday by "playing" some images. In this case, I did not remember to change the target zone in jremote and the images were displayed on my device and also on the server (not clear why this happened). In other cases I have selected the "this device" option and it has still played back on the server.
with an MC client, I have a vanilla linux mc client connected to the server. By vanilla I mean it has a single zone named "Player". It is configured to not be able to target the server zones for playback. I don't see any way in which this Player zone could be linked to the server yet it will intermittently trigger playback on both the server and the client. The only consistent behaviour is that it only happens the first time I click play, i.e. if I stop playback on server and client and then play again, it plays on the client only.