The CIA doctors or whoever did a cursory examination of him when he was returned, also probably provided with his medical history, knew the DPRK didn’t cause this, so they advised the family not to do an autopsy because it would prove them publicly innocent.

It reminds me of how in some elections in south america, the US will instruct the opposition parties to not run candidates (ie refuse to participate), if they know the left wing party would smash them in popularity.

Ie don’t do your enemies any favors by making something public that likely helps them and hurts you.