Just to clarify, I've seen lots of hackdays organised by platform providers so they let you keep the end result and they still benefited because their platform got traction/apps/focus/whatever but never one where they actually take your work off you.
Thanks for that, it makes the original statement "...if a participant creates a blockbuster app that the company can market" much more clear: it's not a marketing gimmick for the app but some marketing gimmick for a platform for designing apps, which the company happens to be selling.
At the same time, it confuses me even more, because I'm surprised that this works as a marketing tactic. ^_^;;