^^Kidnap is the unlawful taking of someone. Now, the Thalmor occasionally do this, but they have to do behind the Empire’s back. I’d imagine only between 1-5% of all Thalmor prisoners are imprisoned illegally (kidnapped). Most the time they have to gather evidence of Talos worship and present to the Imperials or their prisoner has to be let free as per their rights as an Imperial citizen.
In a lot of ways, the Concordant actually restricts the Thalmor as well. You think they only want to capture and interrogate Talos worshippers? We’re talking about people who want to destroy the Empire, take over Tamriel, establish Altmer supremacy and possibly (if the theory is correct) undo creation.
I’m not saying everything they do is within the treaty, but if they are found to openly violate it then they would be in trouble with the Empire, which I honestly don’t think they want right now, we can see they want to cause unrest in the Empire. I’m not going to say the Concordant is a good thing, because it’s not and the Thalmor aren’t ‘good’ people. They are however a powerful nation that the Empire can’t ignore and have to have some sort of agreement with as they both train armies for the next war, which I can guarantee is one of the reasons the Legion in Skyrim is so poorly equipped compared to the rest of the Empire