The alternative is requiring 24-hour moderation, which isn't really feasible unless moderators are paid employees, or just having to deal with posts staying up until a moderator/admin comes online and can sort them out. Communities can obviously try to have a mod team comprised of people from a range of time zones to increase coverage, but aiming for 24-hour coverage would make most mod teams far larger than is particularly necessary for the size of most communities at the moment.

Posts being removed and flagged to moderators for review if a certain report threshold is met is the best middle ground for a community-run, non-commercial forum. Sure, someone can set up 10 (or however many the threshold is set at) accounts and report a post on all of them to have it removed until a moderator is online, but is it really worth it to go through that effort just to get a post taken down for a couple of hours before it gets reinstated?

It's the best way to allow the community to self-moderate, I think, rather than requiring all the moderation power be in the hands of those with a moderator role.