First, bias happens. A journalist should aim to be objective but they are not saints of objectivity. Second, it's not unreasonable to editorialize a little bit when discussing statements that appear to a reasonable person to be made in bad faith or are self serving. If I routinely made claims of fact about the honey-like nature of the heavens despite frequent demonstrations of my claims' absurdity and falseness, it would not at all be unreasonable or "biased" to point out a priori that I still had not produced any evidence of my claims. It's certainly not unreasonable if I run late night TV commercials asking for money from pensioners to launch a mission to collect sky honey.

As for evidence of an event occurring, you'll also notice journalists go out of their way to second source statements of facts. They want first-hand accounts from named sources in order to put an identity and reputation up for scrutiny. Ideally they also want a second named source but failing that they will often accept unnamed sources (to whom the journalist knows the identity but does not publicly reveal it). This is a common practice specifically because journalists are rarely in the room when and where events occur.