Learning champs on ranked?

I don't understand why people can play ranked games with champions they have not even on first mastery. Another ranked I had a guy who took champion he never played with because "it's only flex". Why people don't go with this kind of stuff on normals? Also, can we report someone who picked a champion he can't play with? How it's different from trolling? Why there is no rule that you need to have at least second mastery to play this champ on ranked? It's not hard to achieve it. It would also help Riot with boosters, who can't pick their "mains" on someone's account.
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