It can be, but it isn't always. The reason for the presumption of innocence combined with the proof beyond a reasonable doubt standard in criminal trials is a judgement about the desirability of the particular effects that can be expected with and without that principal.

It cannot automatically be assumed that those considerations generalize to other contexts -- they must be considered in each context.

In the broader context, like in the legal context, it's not a claim about what is true, but rather a claim about what we should believe (or more appropriately, how we should treat the accused) given the evidence we have. You don't say "this person definitely didn't commit a crime, because we don't have evidence," but rather "we don't consider this person guilty, and thus don't punish him or her, until we get some evidence."