Doug Otte wrote:
Those situations always bugged me, too. Both sides would have been prohibited from entering the zone.
The show was notoriously inconsistent about this. In the episode "The Neutral Zone," for instance, it's shown that there were actual Federation (and Romulan) settlements there, which were the first to be assimilated by the advancing Borg.
I actually used this in the Borg Spotlight
issue that I wrote for IDW, and yet still heard from people who'd thought it was impossible. I'd say that it's best to regard the Neutral Zone as a territory whose rules grew steadily ambiguous over the decades in which the Romulans and Federation had no formal contact.