A quick search states that "ROW" stands for "Rest of World", and that Borderlands 2 has "ROW" and "RU" (Russia) versions, so it's not a Russian key.
Games purchased outside of Steam can't be added to the inventory, only games adquired directly via Steam can be added to the inventory. So there's no reason to think the copy is "illegal".
The only thing that occurs to me would be them suspecting you using an Alt. account to buy it cheaper from another regio, using a VPN or similar claims.
I don't have any alt account. And never use VPN.
All my games are purchased legally from authorized sites and indie bundle sites. Some games are from through trading. How the traders got their game I don't know.
Anyway, I have sent a support ticket to see what they have to say. I hope I get the game back. Else I have lost Borderlands 2 plus the games I traded away