This is, unfortunately, exactly correct.
I know most people don't have minds that work this way, but there are those that would have their friends create as many dummy forum accounts as possible to rig the vote otherwise... spamming posts to qualify as they go. It would be great if such restrictions weren't necessary, but they are, and you can't run a competition of any sort without taking precautions like that.
Please understand that the above is not an accusation towards anyone in the competition. I have a feeling that most instances of post spamming were done by individual people simply trying to qualify so they could take part, but the only way to stop it from happening on a large scale as a vote manipulation technique would be to stop it on the small scale as well.