This varies widely.
Referback happens to be owned by Carmen Media, which bought it during the contract standoff last year and fixed things. Carmen also owns Belle Rock, which owns the casinos Referback promotes.
Many of the affliate programs are individual businesses who have the casinos as clients. Some are general maketing companies who have the affiliate program as one department. Others handle nothing but affiliates. Still others are owned and tightly managed by the casinos. Sometimes the casino ownes them but gives the managers a lot of freedom.
There actually is some of the “trading websites” type of thing going on, casinos being traded or split up and placed with different programs.
But mostly you have mergers where the two companies combine and throw their lots in together, or acquisitons, where one buys the other outright.
So you see, this all varies alot right now. All kinds of creative deals are being made.