The core thing here is the "I'm sorry you feel this way". This immediately deflects all sense of wrong-doing from the people actually doing wrong to the people feeling hurt. There are so many other ways to phrase this that are either more neutral or even acknowledging of some kind of mistake being made that's not on the volunteer's side, but that's not what's happening here. Essentially this means "We did the right thing and now we need to figure out how to make you understand this", not "Something went wrong and we need to figure out how to come to an understanding which might include us having done something wrong".