These sorts of behaviors are beginning to be punished quite strictly. In some cases, it may not always be clear whether a player is having a bad game or whether they are intentionally feeding. Obviously, when someone goes 0/32, that's not just a bad game. That's obviously intentionally feeding, and those sorts of games, especially if the player's items are trolling (nothing but boots or tears, etc), are fairly easy for the system to confirm as feeding.
As with any other sort of rule violation, the most important thing for you and other teammates to do is to report their actions. It has to be reported for the system to be made aware of it. And while most everyone would be highly annoyed by someone on their team doing this, not everyone actually reports for it. Quite a few players are so pissed off that they simply close the lobby as soon as the match is finished, wanting to move on as quickly as possible. And they have the right to do so, but if everyone on the team does that and no one ends up reporting the feeder, then they are never punished for it.
As The Djinn pointed out, you will only get the alert if yours is the last report that triggers the punishment. In the match that you played with him, if any of your other teammates/opponents from that same game submitted a report and theirs happened to go in a fraction of a second after yours, they would see the alert instead of you. Just because you haven't seen the pop-up doesn't mean that the player has not been, or will not be, punished.
One last thing... We've been seeing quite a few threads here on the Boards from people who were punished for intentionally feeding recently. Because it's a gameplay related offense, and not a chat related one, the first offense punishment is actually a 14 day suspension of the account, which means a second offense in a short time span could result in a permanent suspension of the account. Some of these threads are from people who have literally only fed one time, and they have been given temporary bans. So, it is something that Riot takes very seriously. They have listened to the playerbase when they say that intentional feeding and trolling is one of the most disruptive and game-ruining things they can encounter, and they are responding to that. But because the punishments for it are so severe, they probably do try to make absolutely certain that the players are in fact guilty before they issue a ban.