Sharing accounts is not prohibited. When someone is banned for RMT they'll usually ban any account that person has ever logged into. So chances are if you didn't have anything to do with RMT, someone who did logged onto your account at some point (thus the references to not sharing your accounts).
If your account is clean and was merely caught up in the dragnet banning, submit a petition on the
petition section of the forums with all the appropriate info and it will get taken care of.