I never saw any posts complaining about gaining LP because enemy has AFK/Troll/Feeder
Then why are you complaining about losing LP due to them?
Statistically speaking, if you personally are not a troll/afk/feeder, then your team has a 4/9 chance of getting one while the other team has a 5/9 chance of getting one.
I have to agree that Trolls/AFKers are issues riot needs to take care of, but If you are ok with gaining LP while the enemy team has a troll/afk/feeder, then be content when it's your turn.