Although some may think the 'Key Hidden Here' doormat promotes theft and reduces home security, the doormat itself can't really be the source of blame. Classic key hiding spots such as the potted plant, beneath the doormat or above the door, should instead be changed.
Implications - I would totally buy one of these splendid and comical doormats for friends who are constantly forgetting their keys or leaving it in obvious spots thieves normally search. If you think about it, it's their own fault if their stuff gets stolen. The 'Key Hidden Here' doormat simply promotes honesty and a good laugh.