Stream sniping is the same thing as ESP hacking in my mind, just on a more limited scale. It's gaining an unfair advantage that others in the same game don't have. It's also intentional griefing of other players.
I was watching Summit's stream yesterday when it was happening and Apex and his partner were consistently showing up at the same locations that Summit and Shroud were at that it was obvious sniping to the viewers watching their stream. With the information that Bluehole would have about their queuing habits (like entering and dropping until they get into the same game), and how they moved during the games they were in together (like suddenly changing directions when it was clear where Summit and Shroud were heading from watching their streams), they would potentially have very strong circumstantial evidence of the sniping. I would suspect with how close in time several of the games were, that they were even dropping from their current game after successfully killing Summit and Shroud. Blatant griefing like that shit should be stopped.