The problem comes when people have different opinions on what constitutes a "god awful one sided match". For some people, if the enemy gets 1-2 kills before your team gets a kill, you're obviously doomed. Then they leave, and you're outnumbered, more people leave, you're more outnumbered.There's a huge snowball effect. If they introduced some punishment for leaving a match early, you won't be put into these matches as much, because for the most part, they won't appear nearly as often.
I can honestly say that the most one-sided matches that I've played (winning or losing) was caused by one team continuously leaving. players leave, new players spawn in with no super or knowledge about the enemy team, and then the new players get massacred. Repeat cycle. The best way to stop it is to nip it in the bud: people leaving.