That's not what's happening here though. The equivalent to that film example would be, like, some tour guide showing off the statue and saying, "This statue is about female empowerment," or something. It might get some guff for being an incredibly stupid interpretation of what is clearly not a statue about female empowerment (this is assumed to be before Fearless Girl), but I doubt anyone would consider it immoral, unethical, or illegal.
A superior analogy to what's happening here would be if I magically transformed every copy of Triumph of the Will in existence into a version where I give that lecture at the front. Without the permission of the, in this hypothetical very much alive, creator of the film, or that of the film's distribution company. Every Youtube clip, every original copy, now has my lecture, and, because we're making it as analogous to the bull situation as possible, the lecture needs to fundamentally change the meaning of the film, even to modern audiences, somehow.
I would consider that bad. It's bad for the public because they don't get to have the original meaning of the film, and it's bad for the creators, which matters given that the real scenario features a non-Nazi artist. As was noted way back, we care a ton about George Lucas altering Star Wars, and that's the original artist doing it in a way that creates far less of a fundamental change. Keep in mind here, female empowerment has nothing to do with either the original intended meaning of the statue, the forceful something or another of America, or the meaning often assumed for it, love for bull markets. It's straight up a completely different thing.
So, we gotta assume that our Triumph of the Will lecture is something like, say, "This movie is about different varieties of marching and the metaphorical value thereof." Actually, what would be really funny would be to show off clips of other cool marches nation by nation, and stick Triumph of the Will in the middle of that. That could maybe be sufficiently meaning warping context to get close to this situation. And, again, it is now physically impossible to watch a version of Triumph that isn't the, "Ain't marching cool?" version. Seems much much less ambiguously problematic than the situation you presented, and also much much closer to the true situation we're dealing with.