Agreed - I'm not saying they'd stop you (you're basically doing their marketing job!), just pointing out that it could be a closed up loophole. For example, the more people that are bidding that, the higher dropbox has to pay for their own ad buys... i.e. if everyone is doing this, they either stop doing the ads themselves (unlikely, as they can't monitor) or stop the practice so they can control their trademark searches (for optimization, etc)

Anyway, not saying what you did was bad - I think it is really cool!