Going to see Tin Man isn't "expansion", though. At most, we learn that Romulan ships can slip past the Federation siege, which isn't really a miracle considering that they can cloak and that part of their realm borders on non-Federation turf such as the Klingon Empire.
Seeing how difficult if not impossible it is to prevent Romulans from crossing the RNZ, though, one would think Starfleet would have every incentive to fall hard on any ships found outside the Zone. There isn't likely to be a loophole in the RNZ treaty that would allow ships to exist on the wrong side unless the act of crossing can be verified... At least not after the first few incursions! So conversely, any ship found outside the RNZ is automatically guilty of crossing it, even if said ship currently is found in neutral space. Picard should be compelled to set an example every time he met Romulans outside Romulan space.
Then again, in many of the cases where Picard runs into Romulans outside the Zone, those are committing other offenses already, such as firing at our heroes, so the treaty violation issue need not be separately discussed. And he has received at least two genuine defectors, so shooting everybody at sight is not necessarily a good idea. But episodes like "Timescape" make one wonder... Why didn't Picard arrest the Romulan crew when no excuse for their presence in forbidden space was forthcoming?