It is really tough to get murders, thieves, and thugs to give up their hostages.
[southfront.org]
I agree with @FuzzyMarineVet completely. I am also led to believe the plant also contains a large number of high-ranking NATO trainers and commanders and the Russians want them all alive.
The Russians plan to have a war crimes trial this summer and they want these mercenaries alive to testify against their handlers (Canadian General, French Colonel, several US 'specialists' and a UK weapons manufacturer representative are said to be among them)
They know they have done enough evil to warrant being punished, but they don't want to take responsibility for their actions. So they hide behind squishies to keep from being brought to justice.