I wonder if part of the reason is because the direct source was provided, it'd be not only much easier for Google itself to find and censor the image, but to prevent mobs of Internet users trying to "doxx" or otherwise prank businesses/residents of that location.

Obviously, Google probably doesn't need much help in performing a reverse image search across its own street imagery. However, the additional bad PR that comes from anonymous users targeting a locale would most definitely be a strong incentive for them to censor a particular Street View image.