is sharing an account against the rules?

Yes, it's against the TOS/EULA and will get the account perma banned. The below is from the TOS and Riot's posted about this quite often.

C. Login Credentials. In creating an Account, you will be required to select a unique username and password (collectively, “Login Credentials”), which you will use each time you access the Game. You may not share your Account with anyone other than as expressly set forth herein, and you are entirely responsible for maintaining the confidentiality of your Login Credentials and for any and all activities (including purchases and charges, as applicable) that are conducted through your Account. Please notify Riot Games immediately if you become aware of any breach of security, including any loss, theft or unauthorized disclosure of your Login Credentials. D. Account Sales. The Account supplied to you is personal to you, and Riot Games does not recognize and expressly forbids the transfer of user Accounts. You shall not purchase, sell, gift or trade any Account, or make any such offer, and any attempt shall be null and void. Any distribution by you of your Account and/or your Login Credentials (except as expressly provided herein or otherwise explicitly approved of by Riot Games) may result in suspension or termination of your Account.

Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes. Yes.

Please read the TOS/EULA.

why is that? if you allow someone to use your account the account should not be banned

Short answer, yes.

If you wanna share? Cool. Use it at your own risk, that's all.