It is the old argument:
- Do I ban the champ that is meta and someone on the other team will probably pick (and will be a royal pain in the ass to face)?
Or
- Do i not ban it because a team member wants to play it?
Usually I don't ban something that someone wants to play but there are times where certain champs are too op to give any leeway for the other team to pick that champ (chogath for example). - A tactical choice...
The main problem with limiting the banning phase like that is because it'll not allow for the good tactical aspect, and league is not supposed to be built around players being trolls to each other (it's built as a game, as such its gameplay aspects shouldn't be restricted)... that's the reason for a separate punishment system (which may or may not work appropriately - I have no actual clue).
In all honesty adding your idea as a feature will only make trolls find something else to exploit to annoy people. This is a problem in itself because if riot decides to keep restricting features to avoid any and all trolling it will just ruin the normal league experience. + if they cant troll you by banning your pick they can troll by not allowing you to ban certain champs and making it easier for the other team to get that champ (or just be an in game troll which is many times worse).
with the issue you bring up you have a choice of over 100 other champions (or however big your pool is) which allows more room to wriggle. If the reverse occurs (stopping you from banning an op champ and then leaving it for someone to take) you or one of your team have to play that champ if you don't want the other team playing it (I'd find this way more frustrating).
I'm not trying to prove you wrong I'm only pointing out factors that should be taken into consideration when fixing an issue.
Side-note Riot has been aware of this but issues with implementing a change (similar to the thoughts above) hinder that and as such it is delayed as the work needed to do something about it and the value of the change is not equivalent.
Now we don't have to give in to trolls but we've got to take things in stride otherwise we can't just enjoy the things riot gives us.
tl;dr Issues with trying to implement a fix to this + the change compared to the work is not equal in worth + trolls are slippery blighters
Oh! And set me alight with the flame of your scorn... or something idk