I've been to several multi day and one-day hackathons, mostly in the Bay Area. I've shipped code at every single one, but I definitely share your frustrations at those who turn up and demo stuff they built elsewhere. I also have had trouble actually socialising at some hackathons, because everyone was so focused on their own project they didn't want to talk for 5 minutes about their problems (related to the app I was building to solve them). Oh well!

I think clearer rules around "look, guys, congratulations for doing a startup, but this is not the place to pitch your year-old app" would immensely help.