The reason that a Soul Binding Contract was offered was obviously because Roan wasn't in the Void Tempering Realm yet. That meant the contracts still 'worked' on him. He still had three stages to go, which wouldn't be surprising if he took hundreds of years to achieve.
Of course, the same couldn't be said for Luan. He had no way to avoid Soul Binding Contracts. With that said, Roan sent him straight into the Dimensional Realm and would be able to travel with the guy. Roan had no intention to meddle with anyone's struggles. It's just that he couldn't predict the future, so he took Luan out of the equation and didn't let the youngster sign the contract.
Kentucky, on the other hand, was affected by the immunity against Soul Binding Contracts thanks to the connection with the twins. Roan had him sign the contract in case he needed Kentucky for some flight in places where identifications were necessary. "In any case, I could swear you would be some fat guy, Kentucky."
