An audit log is currently not a priority right now. In the future, we'll most likely have one that extensions can hook into themselves (do not quote me on this, there are NO guarantees that any feature will make it into core, specially with the view we have for a simple base).
Until we get to that point (if we ever do), this extension is an alternative to that potential core feature. Similar to my admin dashboard extension - while that one is not paid, it sprung from a core issue about revamping the admin UI.
I understand people want many features to be included in the core software and/or not have to pay for them. But we have to remember that we can't include too many features in core or we risk over complicating it and making it less extensible. We also have to remember that Flarum core is not supposed to be your regular forum software.
I'm not saying we should or shouldn't have an audit log in core - I'm just pointing out the reasons why we don't yet and may never do.