The Geneva Conventions specifically exclude mercenaries from the protections granted to captured soliders, and these guys fit the criteria for being a mercenary to a tee, like the only defence they could have is that they weren't paid or were paid subsistence wages, which I highly doubt they were. So Russia can legally try and even execute these guys.

Also, the US just straight up declares anyone it's fighting to be irregular combatants which also deprives them of any Geneva protections. Like they were murdering POWs all the goddamn time in Iraq and Afghanistan, indeed some of these captured US veterans may have been the very ones doing the murdering.

Honestly, it's quite gracious of the Russians for even giving them a trial, the US was never so generous to the people they captured.