Well theres two ways of looking at it.. if you don't set limits, people will play the players too loud, and then sue when they develop hearing loss at an early age.
And if you do set limits, people whine about it, then sue when they still play the players at max volume, for hours at a time, and develop hearing loss.
Damned if you do, Damned if you don't. Generally the public are NOT going to listen to any warnings until its too late. I still see people coming out of concerts raving how awesome it was cos their ears are ringing.. I say limit them all to 89db (Plenty loud enough and the Replaygain standard), but then I value my hearing and that of future generations.
If only all players had IEMs rather than craptastic ear buds, then we wouldnt have to turn them up so much. I use W3's or ES3X on the bus, and I cant hear the engine noise, and never have to turn up past half volume.