That's simply how diplomacy works.
Is it? In a environment of many dozens, perhaps over a hundred alien species,
all with their own cultures, traditions and histories, you honestly think the rules are going to be exactly how they are on Earth in the early twenty-first century?
ALL the rules? EXACTLY like they are today? No, of course not.
But he is an "ambassador." Star Trek's producers and writers generally don't take a real word with a specific meaning, and just arbitrarily alter that meaning, and then not tell us. The use of the word "ambassador" suggests that such a character functions in the same way as real ambassadors do, until and unless we are informed otherwise. Since we never
were so informed, the assumption is that it means the same thing that it does today.
In addition,
Mr. Laser Beam is correct in saying that "that's how diplomacy works." We saw diplomacy in action a HUGE number of times on Star Trek, and there was every indication that the Federation conducts itself in a manner that is quite recognizable to us 21st-century humans.
While we're at it, maybe we can stipulate that the Federation leader can't ask the representative of a usually hostile power whether it's advisable to send a armed task force into the Klingon Empire to rescue two captured officers. That certainly isn't exactly how we (Humans) do it today.
If you are referring to the scene in TUC where the Romulan ambassador is inexplicably still present when West outlines his plan to rescue Kirk and McCoy, that is a major writing/production gaffe, and I honestly don't see what it has to do with this discussion.
This is Star Trek my friend, outside of canon, everything is open , and everything is on the table.
No. Not everything.
This is the antithesis of the point I was making in my previous post. Star Trek's setting is not simply a free pass to just throw ANY idea out there and have it stick. The setting DOES have established rules and parameters, and while some things do work in an entirely different way than they do today, some other things work almost exactly like they do today. There is every indication that being "an ambassador" means pretty much the same thing as it does today, save for adjusting the scope to a galactic, rather than planetary, scale.
^ There is no evidence that Curzon was from any world other than Trill. Heck, he was even on the board of the Symbiosis Commission, which is located on Trill.
He can be from a non-Member world and be a UFP citizen. There is a thing called naturalization. Worf is/was a UFP citizen even though he was born in the Klingon Empire.
Certainly, individuals from non-member worlds can become citizens. But while Worf chose to study Klingon culture while living among humans, he was a nobody to
other Klingons (until that mess with Mogh and Duras came to light, anyway).
Curzon's status as a well-known, borderline legendary figure was as a diplomat, AND as a UFP citizen, AND as a Trill. He was very well-known among his own people, and odds are he grew up on Trill. He is just not presented as someone who left his homeworld and had to go through this kind of process to become a UFP citizen. And I'd forgotten about him serving on the Symbiosis Commission, but that's a good point. I really have a hard time seeing how one individual could serve both on that commission, and as a Federation ambassador, if the Trill aren't members.