Judging from how Joe dealt with Hunter, it is clear the two men had more of a bargaining relationship than a father-son relationship. If you think about it in terms of how organized crime stays organized by a series of implicit and explicit obligations and payback, it probably won't be too far from the mark. The book is Hunter's payday, in exchange he can out himself as a horrible person, but rest comfortably knowing he never has to work again. Having that second income from public prurient interest also means he now has money that is clearly not from Burisma/China, even if that Burisma money becomes too hot for the family to hold on to. Owning that money also means owning the crackhead bad son persona, but you can bet there is arrangement made with the media to refurbish his image a couple of months from now.

You ask how could someone stoop so low as to do this. But like the mafia doesn't question if their drugs and prostitution money are earned with dignity, this is only about getting paid, by whatever means necessary.