Kidnapping vs international arrest warrant
If Seleznev had been chloroformed and stuffed in the back of a car, it would have clearly been kidnapping. What happened here is much more of a gray area.
The Maldives is a member of Interpol. Interpol "Red Notices", informally called international arrest warrants, provided the legal basis for Maldives officials to detain Seleznev at the airport. It's not clear what legal process, if any, took place in Maldives before handing him over to US agents. Nor is it clear what Maldives law requires in such situations in the absence of an extradition treaty.
The judge in Seleznev's federal trial did not permit his attorneys to argue that his detention and transport was unlawful, so his defense had to use the strategy of casting doubt on the evidence of his cybercrimes. This was ineffective, leading to conviction on nearly all charges.
I expect that the judge's decision now leaves the door open for Seleznev to appeal his conviction. The story is not over.