Please don't organize corporate 'hackathons', as they are spec work of the worst caliber and are exploitative. More info @ http://www.nospec.com
Why would a company run such a hackathon? To find talent. To make a PR splash about something. To deliver an audience for a partner. To generate ideas. There are lots of reasons.
To each their own.
This is why it's always OK to call BS. Sometimes you're calling BS, and sometimes you're spouting it. Debate works until it doesn't, and then you get out.