I don’t think Beeminder really positions itself as something to provide public credibility. It’s a personal tool: you can add fake data every day, or edit all your past datapoints, or add a bunch of fake backdated datapoints, and craft an elaborate web of lies! You can probably even weasel out of all your derailment payments! But all you’ve accomplished is wasting your own time, lying to yourself instead of actually working toward the goals that you could have been Beeminding to make progress on instead.
Given that I often need to edit or backdate data to make my graphs accurate, including data that’s days or even weeks old, it would really break Beeminder for me if we were locked out of editing datapoints, without making the graphs any more “credible” to outsiders because it still wouldn’t actually be Beeminder’s job to prevent you from entering a bunch of fake data in the first place.