It's all about communication. If anyone quits without discussing it with the rest of the team first, it's a rage quit. Period. (Unless someone legitimately loses connection)
Of course, some rage quits are more understandable than others. If someone is unwantedly trolling and being disrespectful, rage quitting is acceptable. It's polite to let the respectful people on the team know first, however, that you are about to ditch because of a douche.
In the case of someone else rage quitting, all person B needs to do to avoid it being a rage quit is to let the rest of the team know you aren't interested in playing anymore since it won't count to leaderboards. Who is going to have a hard time with that unless some other agreed upon objective is at stake?