Physically confronting someone in person about something is never comfortable. In-person social interactions are awkward enough, and then you go and throw the whole, "Hey can you stop doing such and such" into the mix and that makes the prospect of such an exchange pretty much unbearable.
So it's no wonder that people oftentimes resort to the ever-useless, passive aggressive note to get their message across. Yes, when taken too seriously by their creators, passive-aggressive notes/signs are the worst and immediately make anyone who reads them roll their eyes in disgust.
However, if you're writing a passive-aggressive note or sign and make a joke out of the whole thing, or get really creative with it, well, you become a legend, just like some of these folks.