But more to the point, I'm just viewing it from the individual user's perspective. From your point of view, you're actively harming the world by donating to an anti-charity. It doesn't matter that those other wrong-on-the-internet people think your donation is doing good. :)
You want to focus on the individual user, and that makes sense. But to him/her, they're using an anti-charity because they want to increase their odds of accomplishing their goal. So when thinking about whether it is "good for the world" or "bad for the world" that people do this, we have to factor in:
1: the x% probability that the user will fail, and money will be donated, and
2: the y% chance that the user would have failed without the anti-charity option, but succeeds instead because of it.
When you consider both of these aspects, it is not at all clear that the anti-charity option is bad for the world.
Exactly zero harm befalls the world in the latter case.
I'd even go so far as to say you could achieve the same effect with a true charity by risking an amount that you really couldn't afford. But, as I point out in the blog post I linked to above, no one ever has the guts to actually do that.
Btw, on Beeminder we very occasionally have people who risk thousands of dollars. I think they tend to be really hardcore fans who may in fact view it as improving the world by giving us money. Hence them shrugging off smaller amounts and needing to risk something ridiculous to stay motivated to stay on track. But it's still super win-win (because they do stay on track overall -- and sometimes staying on track means things like finishing a PhD!).
Is there any evidence to support this theory? Seems like the existence of tools that offer anti-charity donations indicate that for at least some people, this assumption is incorrect.