This guy was allowed to be a customer and he knew d**n well by signing up what social contract he was getting into.
If I wake up in the morning, and I find 511 BTC in my account, how did I steal it? Please explain that.
By not taking measures to return it to its rightful owner, and selling it as soon as you "found" it. As per the definition of theft in your state's laws.
Once it was transferred to me... I AM THE OWNER NOW. Why is that hard for you to grasp? If a pool sends you BTC for payment, YOU are the owner. Not the pool.
You guys are just hating! I love it! Bring the hate!