Do they really do that?

I continue my occasional series about signage in and around Boston. Here is one recently seen on the #77 bus that runs down Massachusetts Avenue from Arlington Heights to Harvard Square.

I don't recall seeing this sign on other routes, but it may be elsewhere. What does this say about the clientele on this bus route, or others, that such a sign would be necessary? I fear it is not a very nice indication of people's manners.

Maybe we should expect it though, as people have been trained to believe that saving spaces is acceptable behavior when there are snow storms in Boston.