why?
Because Riot doesn't ban trolls. They ban people that use cheats and bad words.
Why is banning someones intended pick not a breach of the Summoners Code?
Bans and picks are absolute. Intentions are not.
If you are not flexible with your intentions, you are always breaking the Summoners Code.
If you are not flexible with your pick, you are always breaking the Summoners Code. (Exception being first pick)
If you are not flexible with your bans, you are possibly breaking the Summoners Code.
The difference is bans are done in an uncertain state. You have no knowledge of what the teams will be. You only know what they could be. So your job is to limit what the teams could be. Because you have no knowledge of what the teams will be, you can not possibly consider the needs of the team.
First pick is in a similar situation. They have to follow the absolute rules of the ban though. They are in a position to pick whichever champion has not been banned. They can not possibly consider the needs of the team.
Every pick afterwards has knowledge of what the teams will be. They will always have the possibility to consider the needs of the team.