>What other possible motivation could there be for such a thing than wanting the attention?

To be fair, convertibles are similar. The wind noise of the open air is very loud when moving at speed, so the stereo must be that loud to compensate. There are some setups that will increase/decrease the volume appropriately as the speed changes, but that's not always the case. So as the bike is sitting at a light, the sound is still blaring at the same volume. Maybe the rider is still an asshat for not lowering the volume, but I'd go so far as saying their an asshat for refuting the concept the sound of the ride is the experience by blasting choons.