why can you still ban your teammates champion?

i play so many games with trolls, and they ban my champion. they have nothing to gain from this, they just ban my champion. my most recent game where this happened, everyone called me a loser for dodging (i dont bow down to 5 year olds so obviously i still dodged). this is made more frustrating by the fact that i play mid, which is a super hard lane to get (due to autofill). why is it a feature? why can you ban a champion your teammate is playing/declaring to play as?
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