The bumpers I am referring to are *not surface painted*. The pigment is within the colored plastic. Many are now made of polyethylene or polypropylene. So if there is adhesive left on the plastic it comes off fairly easily without marring with isopropyl alcohol. OTOH Acetone would partially dissolve and cloud the plastic finish so should not be used.
That said, no I don't like anything stuck on my cars bumpers and agree all bumper stickers look rather tacky.