I can't figure out how Kickback gets around the gambling legalities if they are hosted in the US. They also say they use PayPal for withdrawals which is shocking since gambling is against their TOS.
The idea is that if you compete in a skill based game for a pot of money, it isn't gambling. That said, a lot of the games are about as skill based as Flappy Bird.
I can enter a Magic: the Gathering tournament with a $30 entry fee and 2 players where the winner gets $50. This isn't gambling since it is considered a game of skill.
It would not be legal for a spectator to put money on the result of this match since their skill is not relevant to the outcome.