There are many ethical issues relating to retro-bolting which polarises climbers into two camps, those for and those against.
Climbs are produced by a First Ascentionist (FA) scoping out an unclimbed line, deciding to climb a certain section of rock and ultimately climbing it. This first person then gets to name the climb and give it a grading. The route is then generally recorded in a guidebook for posterity. The climb is defined by the style that this person did the climb in, that is, how much protection did they use, was it traditional, did they use bolts or was there a mix of both.
Climbing ethics dictate that a climber should attempt to complete the climb in the same or better style than the FA. This means that they should use the same type of protection without adding any extra bolts. Retro-bolting is the act of adding additional bolts to a climb that the first ascentionist never needed/added.
Proponents of retro-bolting generally cite safety and enjoyment of climbs by all as their major motivations.
The FA may have put up a climb that has few options for traditional protection, and did not add any bolts for extra protection. This means that a fall at certain points of the climb could result in a ground fall. These type of climbs will generally only be climbed by people confident in their abilities and mentally prepared for the challenge. This means the climb may get very little traffic as few people are willing to risk their life in that fashion.
Climbs should be enjoyed by everyone, and just because a climber is not as proficient as the FA, does not mean that they should not enjoy the climb as well. If new bolts are added, more people will be able to enjoy the climb as it is safer, and the people that wish to do the climb in the original way can just not clip the bolts.
Opponents of retro-bolting cite the adventure aspect of climbing and respect for the FA as reasons for not going ahead.
Climbing is an adventure sport that is inherently risky. A climber should understand and accept this risk. If the FA is exceptionally skilled and put up a climb that was both difficult and dangerous, then that is to be respected. The climb should not be "brought down to your level" by the addition of extra bolts which will ruin the adventure aspect of the climb for someone who is proficient enough to complete the climb in its original state.
If a route is climbed very often then it can quickly become polished. This is where the rock becomes smooth as the friction of people shoes and hands on it wears it away. This can be seen in clear evidence on well travelled routes such as "Saul's Crack" (HVS 5a) on the Upper Tier of The Roaches, Peak District, UK. The retro bolting of a route makes it accessible to more climbers which will result in the loss of quality of the rock through polishing.
The first ascent of a bolt-free climb will have caused no deliberate damage. For subsequent climbers to then deliberately drill that piece of rock; cliff or crag to placing expansion bolts, to bring the grade of the climb down to their level, is unacceptable.
Since the FA has accepted "moral rights" their climb, it is reasonable for that person to allow retro-bolting of their climb. The FA may feel that at the time they ascended the climb, the conditions were right for a scary climb, but now they wish to open the line up to more people and let the route have more traffic.
Some of the climbers in the For camp, may feel that action is needed and bolts are added anyway to the climb without consensus. This will generally result in the bolts being chopped, but sometimes the bolts remain intact and the climb becomes accepted in its new form. This is generally rare.