Having worked for the State Department for 28 years, I understand the difference. However, it was being on a Justice Department watch list that stopped the State Department from issuing the visa. The 25 mile radius from Columbus Circle is the agreed upon standard minimum. Some countries impose a similar limit on US diplomats as part of reciprocity.
Then you might know, do we have a treaty based obligation to the UN to admit diplomats? I understand the UN site itself has some odd status, but they have to get to the UN.