That's not entirely true. There are a category of legal defenses called "affirmative defenses" [1] where you are presumed guilty and have to prove beyond a reasonable doubt something that makes your typically unlawful conduct lawful. Self-defense, insanity and fair use are common affirmative defenses.

I would expect affirmative defenses to be fairly common in trials involving police; there's typically not a lot of argument around whether an officer assaulted/killed someone, just over whether the use of force was authorized.

That being said, I would be more in favor of changing jury instructions to indicate that the lack of body cam footage can be seen as an incriminating factor. Not that it 100% means that they are guilty, but that it can be considered suspicious circumstantial evidence. Much in the same way that you cannot be compelled to self-incriminate, but that juries are allowed to interpret invoking the 5th amendment as incriminating.

[1] https://en.wikipedia.org/wiki/Affirmative_defense