I believe they were told to hand over the SSL private key. I'm not sure everyone using the Lavabit service was a criminal... So, my thought is that probable cause for handing over the master key to the hotel should be a bit more than probable cause to hand over the room key...
That's my understanding as well. Unfortunately, Lavabit used one SSL certificate for every incoming user, rendering it impossible to use anything lesser. So to continue your analogy, the hotel only had one key - the master.
