Can someone explain to me why "GG easy" is ban-able but bm'ing isn't?

What's the difference here exactly? "Gg easy" is like a slap in your opponent's face after a match. It's basically telling your opponent how bad he is and how good you are. Isn't bm'ing the same thing? Spamming your mastery token after you kill somebody has the same effect. By spamming it, you are telling your opponent how good you are and how bad he is and that's why you killed him. I mean, there's literally no other interpretation here excluding high elo games where pros do it as a joke. Can anyone explain why bm'ing is allowed?
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