Kira was an adult with experience commanding a vital Bajoran starbase, Wesley was a kid who happened to be on a Galaxy class ship because his mom worked there. He in no way should have had more right to a temporary field commission than Kira (and I say this as a guy who doesn't particularly hate Wesley Crusher, for the record).
If Janeway never caught flak for making B'Elanna her chief engineer at a time where the Maquis were considered terrorists, then Sisko shouldn't need a treaty to trust Major Kira with the center seat of a starship.
Again, the important part is
citizenship and holding a commission in a
foreign military service.
No one is saying Kira didn't deserve to command the
Defiant. But handing command of a ship over to a foreign military officer is a
big deal, because her ultimate legal authority is not the Federation. At the end of the day, Kira as an officer in the Bajoran Militia works for the First Minister of the Third Republic of Bajor, not for the President of the United Federation of Planets. So, yes, handing over command of one of your most important vessels to a foreign military officer is not something that can be done lightly.
To make a real-world comparison, there are few countries as closely allied in this world today as the United States of America and the United Kingdom of Great Britain and Northern Ireland. We share intelligence, our military officers fight and bleed together, our movies cross over so goddamn much that sometimes it's hard to say whether a given movie is American or British. But if someone were to give command of a U.S.
Los Angeles-class submarine to a
captain of the British Royal Navy, that would be a huge legal issue for the same reasons
Nathan talked about not always being able to give information to allied foreign officers -- because at the end of the day, that captain doesn't work for us. His ultimate loyalty is not the the U.S., it is to the U.K. He doesn't work for the President of the United States, he works for Her Majesty the Queen (through the Prime Minister and Defence Secretary).
Things like this have legal consequences. It is not the same as the captain of a starship granting a field commission to one of their own citizens --
particularly a ship that's stranded on the other side of the galaxy, whose captain's actions may be extra-legal anyway as a function of practical necessity rather than obedience to Federation law. But such a situation has absolutely no bearing on the question of handing command of a starship at an important base over to a foreign military officer.
Mind you, nobody here is arguing that Kira should not legally be able to command the
Defiant. We are arguing that, realistically, it should require some sort of treaty agreement. That is all.