Ro was basically on probation in her first appearance or two, IIRC. Uniform modifications are allowed at the discretion of the officer's CO, and Ro was already in something of a disciplinary thing, so forbidding modifications makes some sense.
While the earring is typically religious and she may have been able to argue for reasonable accomodation on those grounds, Ro specifically wears it on the left ear, which is considered a secular way to show familial heritage while also indicating you don't follow the Bajoran faith.