I don't punish for leaving/afk unless the person was being a jerk about it (for example: like walking around in the fountain, and telling their teammates all game to enjoy the loss). In those cases they've done it to hurt their team, and did it in such a way to avoid leave buster detection.
Leave buster will punish players for leaving games frequently (everyone at some point will get a leave, if they play long enough). You can leave one, maybe two games (? I'm not sure the numbers/points for LB) without being punished by LB, so it doesn't make sense to me that we punish a player for one leave with the tribunal.
Tribunal should be used to punish toxic behavior, the punishment system is more sever then LB imo - I'm pretty sure you can't get a perma-ban from leave buster (I would be wrong). You also lose all your honor for tribunal bans (I would imagine you don't for LB) and the amount of reports to get back into tribunal is lower for previously punished players (not that this points a big deal, but I do think they "decay" period for tribunal is quite long/a lot of games relative to LB).
All in all, I punish toxic leavers, not occasional, innocent dc's, or life emergency leavers.