If the chain had a pin (as opposed to masterlink) they may not have inserted it fully. or they re-opened it. It is just a bad design and why i only use masterlink chains (since it cost me a RD and hanger...).
I don't expect them to take the blame and buy you a new frame. You should have kept the bike as evidence and investigated yourself (with pictures here etc.) You basically charged the defendant with investigating and also made them the judge. Don't expect them to plead guilty.....