It wasn't theft because there were no bitcoins at all. It's a scam attempt in disguise designed to make you lose your money instead.

It's a question of what is the moral thing to do if such a thing happened for real. That's where the community has different opinions. Some wouldn't do a thing because it's not their coins. Others would "give" the original owner some time and then sweep the coins to one of their addresses. But the owner doesn't even know that the clock is ticking. Some users would try to warn the owner that their keys are compromised by depositing a small sum and then withdrawing it showing they have access to the private keys...