If a siteowner exercises his or her right to delete something on a website, it's just not the sort of harm that I weigh very heavily.

That's not really the crux of the issue (for me, at least, and probably not for others). As David Gerard put it, the banning of Roko's post was a blow to people's expectations, which was why it was so shocking. In other words, it was like discovering that LW wasn't what everyone thought it was (and not in a good way).

Note: I personally wouldn't classify the incident as a "disaster," but was still very alarming.