Bitcoin is a money and stealing a money is absolutely illegal actually stealing anything is illegal and bitcoin has a good nice value to how it can be legal.
According to certain theory, if you have coded a contract and there is loop hole in the code, you lose your bitcoin, it is legal.
I might know who brought this theory up. And no I don't agree.
Suppose you find money on the street, will you return it or take it (stealing). The money you found belongs to the same owner, and circumstances made it availabe on the street.
You could give it to the authorities. But since bitcoin is not authorized or no one is owning it then I dont think that will be stealing specially if it is an error of some idiot sending money.