I climb, but I'm not 100% familiar with the ethical considerations personally. I think there are different viewpoints. I think these cover some of the views.
Some people think that bolting a wall "spoils" it ecologically, damages it's pureness. Once you've run a line of bolts up a route you can't un-do it. There's always going to be at least bolt holes or bolts there.
Some people don't think "sport" climbing (where you clip into quickdraws on the way up) is "real" climbing, but "trad" (where you place "pro", or protection, on the way up that you later "clean" or remove) is.
You can still trad climb a route that's been bolted, you just ignore the bolts, but some people might consider it less pure as a result I guess.
Some people think that bolting a wall "spoils" it ecologically, damages it's pureness. Once you've run a line of bolts up a route you can't un-do it. There's always going to be at least bolt holes or bolts there.
Some people don't think "sport" climbing (where you clip into quickdraws on the way up) is "real" climbing, but "trad" (where you place "pro", or protection, on the way up that you later "clean" or remove) is.
You can still trad climb a route that's been bolted, you just ignore the bolts, but some people might consider it less pure as a result I guess.