After the first 30 seconds of game time, how we handle someone leaving a competitive match changes. After determining that someone has quit, we begin a 1 minute timer. The missing player can return during that timer, but if they do not then the remaining team members are now given a choice. Depending on the match situation when the player left, that team might want to play on since they still have a chance to win. If the situation is hopeless, we don't want to force players to stick around while the other team finishes the match. So we allow the rest of the team to quit the match without a leave penalty, but they still do receive a loss.

Why do the remaining team members still receive a loss? How is this fair? The unfortunate answer is that the alternative would be worse. We don't want to create an awful situation where players who think they're losing are now encouraged to do whatever it takes to get someone else on their team to leave. By removing any possible incentive for anyone to "tactically" leave, it also means that more matches will complete normally. When someone leaves, Overwatch is less fun to play for everyone.