It's a tradeoff. If you want general-public-scale levels of traffic, you will be subject to the general public's social norms or face their wrath.
The general public did not use Usenet; the audience size was small, relative to what we see today on the WWW. Same goes with most personal blogs. And back when Reddit was not well known, they did not have to have a policy like this.
I don't think centralization has anything to do with it--if your public blog suddenly got as popular as Reddit, I bet it too would be the subject of campaigns about its content.
The general public did not use Usenet; the audience size was small, relative to what we see today on the WWW. Same goes with most personal blogs. And back when Reddit was not well known, they did not have to have a policy like this.
I don't think centralization has anything to do with it--if your public blog suddenly got as popular as Reddit, I bet it too would be the subject of campaigns about its content.