The US Secret Service is chartered with two utterly unrelated duties:
1) Investigation of financial crimes such as counterfeiting and fraud.
2) Protection of the US protected class of untouchable leaders, as well as visiting foreign dignitaries.
I don't see violation of the rights of third party nationals in foreign lands anywhere in their charter. Surely there are normal cooperative channels to bring the case to the attention of the law enforcement agencies of the foreign lands and also the third party governments.
Violation of the sovereignty of the US by attacking it or its citizens does not seem to be a part of this case.
1. This falls clearly under #1, investigation of financial crimes.
2. He was indicted in 2011. If he were, say, a UK citizen (for example), the US would have put in an extradition request, and the UK would have (following a hearing, assuming there was credible evidence) extradited him. Same if the alleged crime had taken place in the UK, and he were a US citizen in the US.
3. Russia doesn't extradite their citizens, period, and, even if they did, there's no extradition treaty between the US and Russia. Therefore, no, there aren't any "normal cooperative channels" involved.
4. If the Maldives government (and I have to assume he was arrested there, otherwise he'd be a complete idiot, knowing that he had been indicted in the US, to visit Guam) consented to his arrest and transfer to the US, that's entirely kosher. The Maldives doesn't have an extradition treaty with the US (they don't have one with anyone, as far as I know, which might have been a reason that Mr. Seleznev decided to vacation there), but that doesn't mean that they're not allowed to extradite people, just that they're not obliged to.