This seems like a very Russian approach.
1. They were unsure of the loyalty of Donbas, so they sent them to die, similar to how they used Ukrainians and Lithuanians in the Soviet Army.
2. It builds the story – even if at gun point – of Donbas fighting against Ukraine.
3. It maintains the superiority of the Moscow / Saint Petersburg Russians – who are strong and don’t have to fight. Meanwhile, the conquered people in the provinces have to go fight and die.