I have seen some with semi-permanent adhesive. (I know that because someone tried to peel them off) If you have easy-peel stickers, I agree it's harmless fun.
Heat makes it a snap for every sticker I've tried it on. In summer, park with that side of the car facing the sun, in winter use a heat gun or a hair dryer.
I agree, though, that bumper-sticking someone else's property (regardless of ease of removal) is stooping to a low level, akin to spraying graffiti on someone's property. (Although I've seen some wall painting which reaches the level of fine art, presumably with permission.)
A while ago i was just about to suggest we use the acronym "LGB" to surreptitiously indicate "Let's Go Brandon," but a little Tinkerbelle-like fairy fluttered by and advised me that maybe that wasn't such a good idea.
Thanks, Tinkerbelle. I owe you.