Riot complicit in abuse
To put it simply, Riot Games condones player abuse. In nearly every single game there is at least one person on your team that flames, trolls, spam pings or somehow otherwise intentionally abuses another person, usually on your own team. Multiple this by two teams and it's a near guarantee that at some point in the game one player will abuse another.
Riot's response is to ignore them in your chosen manner and play your own game. It's the victim's responsibility to endure and/or compensate for the offender's abusive behavior. The report system is completely ineffective for all but the most extreme cases. This doesn't even include the behavior in champ select.
Below is behavior that Riot feels is acceptable based on the fact that it is reported in an overwhelming majority of games but action is only taken in a tiny percentage of cases:
mid ur trash jg you lost the game for us because no gank Surrender fails so sits in fountain flaming team. you're complete garbage uninstall WHY DIDN'T YOU FOLLOW UP YOU F'ING IDIOT YOU MADE ME DIE you fed my lane gg uninstall why? WHY? WTF WHY??!?! gg you lost the game because you're bad at least i have positive score you're trash
The list goes on and is usually much more offensive and abusive but anyone who plays knows this. These same players are regularly toxic in nearly every game but Riot feels that their behavior is fine as long as it doesn't cross a line which usually involves slurs or 9x report.
Every season there comes a point where I simply can't enjoy this game anymore because starting each game I know there's a 90% chance that someone is going to make the experience worse because they want to vent their own frustrations by intentionally abusing another player.
Given the scale of this problem and the fact that it is ubiquitous and persistent from season to season it's a simple conclusion to draw that this abuse is acceptable in Riot's enforcement policy. Consistently toxic players no this too because they even brag about how much abuse they can get away with game after game in chat without receiving any repercussions.
Riot could easily enforce their ToS and at least make players feel that any behavior is allowed as long as there are no slurs but they choose not to so 9/10 games will continue to have these players.
Riot's attitude towards moderately toxic players is the virtual equivalent of telling someone that is in an abusive relationship: "Well, at least they're not beating you, just ignore it and do your own thing"