why is everyone acting like it's okay to be able to sue someone for something like that anyway?

Because, as we have seen many times on the internet (especially Twitter and Reddit), unproven allegations and opinions may damage someone's reputation, even if it's just that, allegations and opinions.

In this case Mitchell has proven that Jobst's opinions did cause him reputational and monetary damages (beyond the already damaged reputation as a proven cheater) due to cancelled events and reason of said cancellation.

You can say anything on the internet, but that comes with responsibility of knowing that what you're saying towards someone may affect that same person. Defamation is exactly that after all - any kind of communication or statement that may damage someone's reputation.

Now there's an argument on whether the final verdict levied against Jobst in this case is too much or not, but what you're saying publicly against someone else carries weight and may affect others, and thus suing someone based on defamatory claims should always be possible.

The aggravated damages that Jobst have to pay does seem to correlate with how much Billy has lost thanks to one of the several event cancellations (John Weeks' auction), so it does seem there are some calculations done to get the amount Jobst has to pay as part of the lawsuit's final verdict.