Perhaps the WorldCon could claim breach-of-contract and sue to have their money refunded. That might be a possible avenue they could win.
Even if they win, the refund in this case would be ZERO because that's exactly how much WorldCon paid to ustream for streaming the event. There was no special contract with ustream, WorldCon CHOSE TO USE a free anonymous streaming account and that comes automatically with copyright-enforcement protection, this was explained by ustream on their blog:
As background, our system works like this in order to support a large volume of broadcasters using our free platform. Users of our paid, ad-free Pro Broadcasting service are automatically white listed to avoid situations like this and receive hands-on client support.
translation: since WorldCon was not white-listed that means they decided to stream the live event without signing and paying for a dedicated contract with the broadcaster and as a result were applied the regular copyright filter that regular anonymous broadcasters were subjected to.