It is illegal and vandalism: there is NO right for private citizens to adhere or glue something, which then can require significant effort to properly remove without leaving a residue or causing damage (e.g. scratches as you scrape it up). He wants to print up fliers or sheets of paper and tuck them under the windshield wiper--that he can do (though if he is caught doing this, he should not be surprised if a car's owner punches him--which would be illegal and assault, but hardly still a good thing to happen). But adhering something to the car is illegal.