Q: My environmentalist friend likes to put bumper stickers on Hummers that say, I'm Ruining the Environment—Ask Me How! I know she should stop, but doesn't a gas guzzler do more harm than a bumper sticker?

A: What a terrible argument! Defacing someone's personal property because you don't approve of their priorities is simply vandalism and harassment, and accomplishes nothing positive at all.
— Jack Marshall, president, ProEthics

What's wrong isn't the bumper sticker—which is pretty funny—but the smug self-approval and feel-goodism behind it. If she has even a small car, isn't she also wrecking the atmosphere? Or suppose she's pro-choice: Is she okay with someone pasting a pro-life sticker on her suede jacket? You don't deface property to trumpet your ideals.
— Rushworth M. Kidder, founder, the Institute for Global Ethics

It's not just wrong, it's what we lawyers call a tort—specifically, a trespass to chattel. Your friend owes a lot of people for any damage done to the cars she's chosen to "decorate." If she feels like interfering with other people's property, a flyer under the windshield wiper might be a better form of protest.
— Anita L. Allen, professor of law and philosophy, the University of Pennsylvania Law School