The problem is not that people like Russia and would get mad at us. Or think that it's "unfair". The problem is that it's a broken promise. An abuse of a position of trust. If we have a multi-century long reputation of "when you come to negotiate with us in good faith, we will guarantee you safe passage for the duration of those negotiations", then people will come to negotiate with us. We don't like Putin, but we would like to stop the war, and being able to talk to him increases the chances of that happening. If Putin thought he was going to be arrested on arrival, he never would have come in the first place. Only because we, and every country like us, has operated this way for hundreds of years is he willing to come to the table to talk. If we go "Surprise!" and arrest him anyway, it'll be too late for him. But nobody else will ever do that again. If we end up in a war with China, or just in tense negotiations that might threaten war, they won't want to come meet with us to talk. If someone in the Middle East starts causing problems for us, they won't trust us to come and talk. Nobody who we dislike will meet with us, or surrender, or offer compromises, if they believe we are the kind of people who will exploit any vulnerability and just arrest them.

It's not about what people deserve, or what we could "get away with". It's about reputation and trust, especially among people we hate and who hate us. It's still useful to be able to talk to them.