We assume "not guilty" = innocent. It's common everywhere, and DU is no exception.

The OP requires negative evidence and that can be tough. Evidence of non-collusion looks like no evidence of collusion.

There are two ways of proving a negative. One is to examine every instance where something might have occurred and to show it didn't. The other is logic-based and relies on proving something about an entailment--that something logically impossible if the proposition is true does actually happen, so the proposition is false.

The first is considered an informal fallacy. Prove there are no magical unicorns. Have you checked everywhere a unicorn could be using the required methodology and in such a way that the unicorn couldn't escape detection? No? Then do we assume they exist?

The usual way to avoid the fallacy is to assume proof of existence, not non-existence, is required. Then to assume that if those best in a position to know if something's existence say they have no such proof, to accept their word at face value.

After that we get into discourse pragmatic and when or why we attribute ill will and uncooperativeness to others.