The oath was acceptional to previous members and he decided to take it as a sign of respect.O_-
It does if you're a sage.
As you should know, the fire sage before Eriasu (he died 5 years ago), was a member of the order of the sages before that rule was passed, yet he took an oath of poverty.
Why do you think he ate at taco bell, giving him horrible gas, the night of his accidental death?