Kawhi was a free agent after the season when he got traded to Toronto, while Harden still has 2 years left on his deal.
Houston could get more for Harden based on the fact that he has more years left on his deal. Harden can't say no in where he ends up as no trade clauses don't exist in the NBA. If he chooses to sit out willingly, then that's an extra year added to his contract.
That's why Houston has all the leverage as he's not sitting out the season. The moment he tries to "fake an injury" will be the moment the NBA comes calling his phone especially since people know what he's trying to do.