If someone put a sticker on my paint they would be in for trouble. Bumper stickers can damage or even ruin the paint beneath them. Older cars had unfinished metal bumpers and didn’t risk damage. Newer cars tend to either be covered in paint, or have unfinished plastic cladding. Both paint and cladding can be ruined this way, and the process of removing the sticker safely (if possible), then cleaning up mess, re waxing or restoring the finish such as with clay bar and detailing, can cost hundreds of dollars and or take hours of labor to repair. It may seem funny on the surface, but like many pranks, the laughs aren’t proportional to the possible damage. He’s an asshole.
He's definitely an a-hole, and if someone ever does this to your car, here's a handy trick: use a hair dryer. After a few minutes of heat, the sticker will just peel/fall off without any damage.
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This guy just places stickers on peoples cars, some of which could ruin them socially.