-"Opening multi accounts is legal" says in the forum rules. Why is this given negative? Would it be a problem unless it signed the same signature?
Forum rules don't forbid scamming either. Trust ratings are not for enforcing forum rules. Forum rules are enforced by moderators. Trust ratings are for some of the things that are not directly covered by the rules, such as scams, attempts to scam, successful trades, not so successful trades, etc. And yes, that might include deceptive sockpuppeting if it creates a risk to other users.
-The purpose of the multi account is: "expressing yourself more comfortably". How can I express myself more easily if I say "this account is mine"?
I don't know where you got that definition but alt accounts can be used for anything. As long as you don't cheat or scam or troll or evade a ban etc - you should be fine. Your colleagues from the Turkish board got caught cheating. "Expressing themselves" wouldn't have involved cheating bounties, vouching for themselves, etc.