I would not call the exploitation of posted, public offers as devious or underhanded. Exploitation of a public offer by a large entity is opportunistic and if abused, perhaps, abusive but not devious.
However, it probably would have never occurred to me to jump on this deal.
But, in my opinion, taking advantage of a stupid person (not a big entity) would be devious and underhanded.
Yes, there is always the law of unintended consequences when you offer something new. People will figure some way to bend it to their advantage. It takes a lot of careful though to prevent such loopholes.
Which is one reason the health care bill scares the heck out of me.