Vonnegut wasn't a Too Good For This recluse like Pynchon or Salinger or whoever, and the content of his books made him fairly well insulated against charges of being a sellout; they probably offered him a bunch of money and he shrugged and said sure.
It was the sellout part that surprised me. I just wouldn't ever expect him to advertise anything. But I suppose as cynical as he was, he just took the money and agreed.