Why is a paceline acceptable but taking a draft off of a car any different? They're both assistance which allows a rider to go (often significantly) faster than they could alone, even under the best conditions. If it's perfectly acceptable to soft pedal for 15 minutes between all-out 2-minute sprints, then it's equally fine to tuck in behind a city bus as it goes by. I don't think I've ever seen anyone out on the open road actively motor-pacing, though I imagine it happens somewhere.

As one of the guys at my LBS said, "Over a long enough timeframe, every KOM on the road will be set by a paceline of 5+ guys with a 25mph tailwind."