not attending because emotionally or mentally the thought of doing so is too much to handle. This does not make someone immature and selfish.
You're right. They could also be behaving either immaturely or selfishly, not necessarily both. "Too much to handle?" We do mature and/or unselfish things all the time because it is the right thing to do, even though it could feel like "too much to handle." Visiting a sick friend, going to a job we hate, flying, getting on the freeway, paying the bills, doing our taxes, saying "I'm sorry," listening to someone who is hurting - getting out of our comfort zone to be there for someone else. Or you could just go "party" because that is easier. And it can always be rationalized - "it was too much to handle - you see I am too delicate and sensitive to handle other people's tears of grief - it upsets me."
BTW, please note that I am labeling the behavior not the person. It is an important distinction. I'm sure that those who have expressed solidarity with sparkplug's decision are not "selfish and immature" people. I try not to label people because we all behave differently in different situations.