Sure there's this principle but this just moves the need for justification. Humans leave their traces everywhere so why this principle for mountains? There are many traces that should be removed such as hydroelectric power stations, river straightening and so on. Is this whataboutism? I just think a few bolts in a mountain don't do much harm and as a casual observer of these mountains you won't even notice. Also I am very much for "leaving no traces" in the sense that everyone picks up their trash.
I am all for free climbing. But I don't really like this perspective from a high horse. So it's fine for comparatively rich people to fly there by plane to free climb Cerro Torre?
It's about leaving it in the state you would like to have found it. No rubbish, no abandoned equipment, no bouldering-style puzzles already solved because someone has stuck a bolt there.
I do wonder whether the boulderers find it annoying when a boulder is clearly already solved by the worn, chalk marked hand holds. Probably explains why many seek out new untouched sites to solve first.
Because it breaks down if we use another example. Would you want to solve a rubik's cube that was pre-turned so there were only 3 flips left to complete the cube? Changing the state of the activity (adding bolts/belays/ladders/etc.) is not in the spirit of the event, just as almost finishing a rubik's cube for someone else isn't in the spirit of actual event.
Put a different way: the climb becomes the bolter's experience, not the mountain's experience. It is no longer intrinsically mountain climbing, it's hiking.
I don't think it's just a matter of ethics - some legal entity owns these mountains (park authority of some kind?) and drilling holes and placing bolts done without the permission of the owner sounds like vandalism to me.
I am of the belief that we should leave a place better than we find it.
When litter is in my path, I remove it. To do otherwise would go against this fundamental principle.
Natural places should be protected from the widespread exploitation by humans. We are destroying the entire planet. Why can’t we protect the places we have long agreed should be protected? Enough with the anthropocentric BS already. We are a part of the world, not separate.
This is why you see in trad climbing the lead will place cams and nuts, while the last in the group on that pitch retrieves them.