Katniss gave Johanna a 'don't fuck with me' sort of look. "I know what I signed up for," she said. Paradise. Not Panem. That was the key. She found a way to escape Panem once and for all. And if that meant she had to whore herself out...well, at least it was on her terms, and not Panem's.
Of course Johanna would see right through her. Johanna knew her. If she'd known Johanna was here...well, she would have still said yes. Because it was an escape, pure and simple. And it was Paradise, right? How terrible could it be, really?