Let's say that Riot responds and addresses this one person outside the system. All because someone pointed them out on the boards.
That's setting a dangerous precedent.
Riot would set themselves up for a huge wave of people constantly posting on the boards attempting to oust other players for what they perceive to be intentional feeding. And they simply wont have the time or manpower to confirm each case. People would constantly point at the few instances that Riot chose and ask why can't their situation be looked at as well.
It's essentially a tragedy of the commons. While Riot employees could try and devote their manpower to help a few individuals that post about their problems, it becomes just a detriment if everyone is doing it.