Stealing is when you take something from him, and he no longer has the thing you took.
Piracy is when you see something for free that everyone else paid money for. You watching doesn't prevent anyone else from watching.
Piracy isn't stealing: piracy only deals in intangibles. Stealing is for finite goods.
There's a whole "how do we pay to make stuff if people can watch for free" problem around piracy, but it's fundamentally a different thing than stealing.