Intentional feeders tend to be a problem with players that get easily angered.
Scenario 1: Your ally dies and you say nothing.
Possibility 1: Ally gets flustered and starts blaming the jungler, mid lane, ect ect.
Possibility 2: Ally continues to feed, but tries to do his best.
Possibility 3: Ally starts feeding on purpose to get attention.
Scenario 2: Your ally dies and you call him a retarded pos and tell him he should uninstall and leave.
Possibility 1: Ally flames back, loses interest in winning, starts trolling.
Possibility 2: Ally mutes you and continues trying his best to win.
Possibility 3: Ally was already going to feed even in scenario 1 and your reaction doesn't have any impact on the outcome.
I would say under scenario 2, his feeding is more or less justified by your lack of control. Now that doesn't make it right, but if someone calls you a bunch of names and acts like a child, you are less inclined to try and win.
Scenario 1 puts the choice in the ally player's hands. If he feeds during scenario 1 on purpose, he is solely at fault and he has no valid argument as to why he fed the enemy and became toxic himself.
In order to reduce the total amount of intentional feeders in YOUR game, you can take steps to avoid confrontations that might escalate into a feeling of righteous feeding, or revenge feeding. It's more logical to assume that by partaking in scenario 1, you can reduce the chances of an incident of feeding occurring by roughly half.
Accusing a player of intentionally feeding is one way to increase your chances of getting one on your team. Highlighting every mistake you see increases your chances of getting one on your team. Being a nice player may not reduce intentional feeding to 0, but it will certainly reduce the overall occurrence of said feeding in your games.