it seems that, in this particular court case, the woman has complied with all the other requirements of the citizenship process (including removing her niqab to an official, so that her identity could be verified), the trigger point was at the final stage of the citizenship process - public oath-taking ceremony, she was required to remove her veil, so that the official could virtually see her mouth to confirm that she's indeed reciting the oath.
According to the judge, this requirement is "superfluous". So this is all about the mouth not the face, an interesting case, isn't it? :)
If you have some free time to kill, you can read the full story here: Baloney Meter: Is niqab ban needed to prove citizenship applicant's ID?