Why is using the mute button reportable?

So, and this happens to me a lot in bronze, i get reported.

This is the common situation. My team is behind. They group. I say, "thats a bad idea, we're behind, and when we're grouped we can get aced, and getting aced is a good way to lose. We also don't have a good team fight comp." They whine and say group more. So i do. We get beat in the team fight pretty bad. I say "see guys, we shouldn't group." They continue to group while i ward, split, and try to focus other objectives. Then they rage at me when they get 4v5'd. I back to defend and while defending they say "Fucking group (whatever champ I'm playing)" or something like "if (whatever champ i'm playing) were with the team we'd be doing better." Or "so useless tefferson, always alone never with the group." So i say again, "Guys, i told you, its stupid to group right now. You can do it all you want, but I've given you my reasons for not grouping right now, and will not be joining you." So they rage at me. They start going into all chat saying to report me. So, i mute them. My whole team, just muted. And then i blame them for the inevitable loss. Then, at the end of the game i get reported for negative toxic and refusing to communicate with team. What did I do and why am i reportable? The refusing to comm, i was using the mute button, i shouldn't be able to be reported for using a function built in to the game. And in what way was i toxic? How was i negative? This is ridiculous that i get reported so much. And that doesn't even touch on the elo hell created by teammates like this.

Can somebody explain this? I tried to say something similar once and got a shitlode of down votes, all that people said was that elo hell isn't real, or that the ranked system isn't broken (which is disputable), and that i deserved the reports, but never explained why in a good enough manner.