I think this is a good way to indicate that your parking is not generally enforced. Submitter Thalassa thinks it might mean that it is enforced by letting the air out of your tires and sending ninjas after you. I doubt it, but I might not risk it either.
I don't usually post the following: newspaper headlines, personal email, craigslist postings, unprofessional websites. I also tend to not crosspost things from other blogs, since I have so much unique material waiting for me to get to it.
Things I see a lot: silica gel "do not eat"; hair dryer labels; inside the bus "do not drill"; Wal-mart sign about IDs; coffee machine with "2" cup sizes; employees must "wash hands".