Coming to think of it… How would you implement user tracking with an image that must be served from a static URL defined in a DNS record and no request parameters to go by? Other than applying heuristics to match the time frame between sending a mail and receiving a request for the logo endpoint, I don't see how that would even work.
Additionally, platform providers have a huge incentive to cache the logos on their end—otherwise, they'd be required to verify the cryptographic signature every single time the logo were required to be drawn on the screen.
