>The premise is genius as marketing you don't recognize as such is extremely valuable.
If that is true, the chances are that the adverts are on the forums that comment on this puzzle, rather than amongst the breadcrumbs of the puzzle itself. Why? Because (I assume) far more people are following the progress of this treasure hunt than are actually participating in it.
There is still the cost/benefit analysis to be done by the marketing team... for the cost of setting up this puzzle (okay, probably far less than creating and airing a TV advert) they want in return either lots of eyeballs, or to market to a specific (self-selecting group). You only want to advertise to a select group if they are a, much more likely than your average punter to buy your product, b, if they are very rich (higher margins), c, you have a way of getting your message under the viewer's radar (as you suggest), or a combination of the above.