From Riot's release Reports: How do they work?
"Sometimes a player attempts to punish another player by reporting them as a form of retaliation. In a system that relies on honesty, that’s potentially a big problem. We can detect false reports, filter them out, and decrease the effectiveness of future reports from consistently dishonest players. As we continue to build more systems that rely on honest reports, the penalties for false reporting will become more strict."
There have been a few people on this forum that I have tried to discourage from reporting players in every single match that they play. Simply because of the above reasons. And like The Djinn says, it isn't even just players who are trying to abuse the system and deliberately being dishonest, but players who earn a reputation in the system for reporting players who are not being reported by other players are going to become less reputable. I know of players who have submitted hundreds of reports in a single year. I probably haven't submitted a hundred reports in the entire time I've been playing. And unfortunately, players like that, even when they are submitting reports for things like hate-speech, may have lost so much credibility to the system that it may disregard their reports entirely. I'm not saying that is a good thing, or the way it should be, simply that it is a potential consequence of their earlier actions by being too trigger-happy with the report button.