Yes, I understand that, but an open voting has a chance to turn into a "who-has-a-bigger-following-base-on-random-social-media" contest. But even if they can't vote, they could still play the game, so you are able to get valuable feedback. And at the end of the day that's what matters anyway, that's why I like that there are no prizes , it's mainly for learning and get to know other devs.
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good point.
And I guess there's always gonna be some cheating in one form or another. But I doubt the few cheaters who might participate in game jams would end up being the talented geniuses who win anyways, as long as the actual best games win. So I'm not so sure chasing them is as relevent as focusing on electing the best games and ideas wich should be the games and ideas players love.