Telling someone not to feed does nothing to help them improve.
Identically, saying, report so and so only makes the team perform worse.
Despite that, these phrases are generally the default used by players who use chat to vent their frustrations.
If someone is feeding they are either doing it intentionally and those cases are legitimately few and far in between, or they are repeating their mistakes and are at a loss as to what to do about it.
Let's say, for instance, a top lane garen is losing to a teemo with full AP, but the Garen insists on rushing BC or Sunfire Cape.
Instead of saying, stop feeding, tell him, "Try building Hexdrinker and let him push. I will roam top to help."
This will bolster Garen's confidence, he will get MR and then Teemo gets camped and loses whatever lead he had. Suddenly, Garen's a fed monster and carries the team to victory despite being crapped on early.
Another example could be your top lane complains they can do nothing against Riven. Tell them, "Wait for her to use one of her abilities before going in on her and keep vision down so you can avoid getting baited. That way you can all in her when her combo is on CD."
Let's also say bot lane is getting shoved in by a poke support and your melee support can't push back. Roam down to their lane and turn the fight around in your team's favor.
These people aren't losing to make you miserable. They are losing because they are at a disadvantage and could use some advice or help to turn a disadvantage into an advantage.