I got over my ignorance pretty quick but to him (and the lesson I took away) is that it is a signal as much as to yourself as to anyone else, is that you are there to absorb what they have to offer.
With regard to ethics - is it non-ethical not to declare your background clearly to all around you. Not at all.
I completely agree that the signal is to myself as well. I did find there was a brief conversation with every training partner at some point as there was the reasonable concern from them that I might not know how to fall or had no framework to understand what I was seeing. Beyond answering that question honestly, no one cared. People were relaxed and open with an outsider and I had a great practice and a great time.
The other teacher from the first story I told - he was lying to everyone he trained with, trying to hold back any information in some cloak and dagger game but he was plainly uncomfortable being treated as a beginner. Even small things, like his partner getting attacked first or giving him genuine accurate points of feedback had his face turning red and his eyes twitching. He would have been happier and still well received if he had just come clean.