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.
Just double checked. Looks like beta-canon from the novels. Interestingly, according to Memory Alpha, the first episode or two with Bajorans in TNG had all the male Bajorans wear the earring on the right and all the female Bajorans wear it on the left, but right ears for both sexes became standard pretty early on. The only other named character that wears it on the left is Lt Mura in PIC.