There's a common conception that anonymity justifies toxicity. I feel this is a flawed viewpoint.
Do you enjoy being verbally harassed by strangers at the supermarket, movie theater, golf course, or what have you? Is that a fun experience for you? Most people would say no. Riot (and other companies like Blizzard, which rolled out new system for Overwatch) are cracking down on this behavior because the fact that it traditionally HAS been accepted in online gaming doesn't mean that it SHOULD be accepted in online gaming. Accepting a standard of abuse is not a good idea for a community: it makes players not feel comfortable with leisure time spent on your product.
The mute button does exist, but it exists as a shield to safeguard against toxic behavior rather than a blinder to enable it. If I have a Riot shield, for example, that's not an invitation to throw stones at me: just a way to prevent injury if you choose to do so. The onus of good behavioral choice is still on you, as is the punishment associates with your actions: I can just ensure I have a way to not be overly harmed by your decisions.