I've gotten in a couple of debates in games before over this topic when people say "report x for leaving" but from the posts I've read on the forums and having reread the rules several times on the Trial System, you can't really do much about them. It sucks and I hate n1 leavers too, but I've also gotten a bit annoyed at people who parrot "report x for leaving" and throw the game for themselves / the rest of their team because the mafioso or an RM left. Reports for d1/n1 leaving come back inno on the trial system for this reason even if I've noticed they've had a pattern for it before, or more accurately, several more gamethrowing reports which I assume are for the same reason.
The rules clearly state if somebody's intentionally going against their role's goal, it's throwing. Which is why the stipulation that they can't really be punished unless they admit to leaving because of their role makes sense, because you can't really prove it was intentional otherwise. Maybe they got a call saying their grandma is in the hospital. Maybe their internet disconnected. Maybe they were typing out their fake will and accidentally hit the X on the window instead of closing the will. (I've actually had that one happen to me, funnily enough, shocker I didn't get reported for it.)
Maybe most of the time they did leave because of their role, but you can't prove it was intentional unless they said that was why. I wish there were a more justifiable punishment for it like James2 suggested, but the rules in place now makes sense. If d1/n1 leaving really bothers you that much, maybe you should just play Ranked. People don't leave Ranked like that in my experience.