I don't see it often anymore, but leveling my alt I encounter it occasionally.
Some people want to troll, and find enjoyment out of making people as mad as possible.
Some people don't want their win diminished, and would prefer to think it was their skill that won them the game, regardless of the afk.
Some people, because they aren't fueled by the anger of having an afk on their team, honestly believe that sometimes DC's happen, and do not want to punish someone for simply having connection issues.
It could be a mix of all of the above, but I find most people to fit into those categories.
I will always report an AFKer. If you have disconnect issues, then most likely it is limited and you will not end up in the tribunal due to repeat offenses. However, if someone habitually AFKs then it is likely they will end up in the tribunal.
I know AFK games suck. For me, it is not a fun game if either side has an AFK. This is what probably fuels me to report every one I see, regardless who's team it's on.