The political landscape has changed dramatically.
First of all, we learned in Star Trek (2009) that the destruction of the Hobus star in 2387 had obliterated the home solar system of the Romulan people (including the planets Romulus and Remus) and effectively removed the centre of power of the Romulan Star Empire. As we can see from even the first two episodes of Picard, the political landscape of the Alpha and Beta quadrants has radically changed by 2399 (the year in which Season 1 is set).
In particular, it is revealed in the second episode, "Maps and Legends", that there is a now a Romulan Free State, which in particular is operating the
Borg cube reclamation project known as "The Artifact".
The Federation has technology and expertise to gain.
The Romulan situation seems to be wholly different from the TNG era. While secretive aspects of Romulan culture persist, the paranoid political and militaristic structures that prevented everyday Romulan citizens from interacting with members of other cultures beyond the Neutral Zone seem to be completely absent. For instance, within the
Borg reclamation operation
we see Federation and Romulan scientists working side by side, sharing technology (as well as other aspects of their lives, as @Machavity remarks).
Therefore, there is already some level of cooperation between the Federation and the new Romulan government.
While the Federation will undoubtedly have security concerns to manage, there is likely much to gain from allowing the seemingly unimpeded movement of individuals from the destabilized remnants of Romulan society — in particular the potential to acquire previously inaccessible technology that Romulan refugees may be willing to trade in order to secure resources and status in the Federation.
Further to this,
Soji refers explicitly in “Remembrance” to the trade of Borg technology recovered from The Artifact by Romulans, including potentially by Narek.
Picard and his Romulan house guests may be under constant (and potentially invisible) scrutiny by Starfleet Security.
As for Picard himself, he is not an ordinary Federation citizen. Given his legendary status, should he wish to have Romulan guests / staff on his vineyard, there is potentially little anyone can do to stop this directly. One has to assume that, at this point, he cares little about Starfleet security concerns and that Starfleet, rather than put a formal stop to his fraternizing, is monitoring him and his Romulan guests for any breaches or unusual activity.
Indeed, despite Admiral Clancy’s call in “Maps and Legends” to Commodore Oh (the current Director of Starfleet Security), Oh was in fact already aware of Picard’s activities, including salient details unknown to Clancy.
Section 31 in particular has a history of letting situations play out to their advantage. It would be consistent with their modus operandi to monitor these two former Tal Shiar agents, namely Picard’s house guests Laris and Zhaban, as a means to gain information and insight into any potential Romulan plots.