For one, everyone who's complaining about lag, it's either you OR the host. If it's the host, it will 4/5 times migrate. If it's you, then as long you're on a one bar, leaving is fine, since you can't get probation for leaving on a one bar, unless it''s the host's connection. And 9/10 times, if it's your connection, you'll get kicked for 'disconnected from host' or 'lost connnection to the host', which doesn't get you probation.
After that, joing a game mid session and leaving BEFORE you spawn is doesn't get you probation, since you didn't play, and treyarch understands you may not like the map. If you leave mid game and were in that same lobbies pre-game lobby, then you CAN get probation, since you could've left before the game even started.
Leaving in-game after you've been playing for a while comes from 1/2 of 2 things:
Not having fun because the other team is too good for you (you may not be bad, but they're better than you),
And you don't want your KDR or WL going down (only prevented by dashboarding) (this is also CHEATING!
If you look at probation and compare it to League of Legends, they give you probation because by leaving, you're giving the enemy team an advantage, meaning everyone that was on your team now gets a loss, and it can make the game less fun. They institute probation so that if you make other people's time wasted by pretty much securing them a loss by leaving, you DESERVE probation.
Long story short, unless it's connection wise, YOU DESERVE PROBATION!