I'm kind of scratching my head at the OP too. If you were so offended why didn't YOU intervene? Yet you whine that others should have and didn't?

You say you're not an employee. My first thought was DAL retiree. But whatever.

And after nearly 2 decades in the airline industry, I have to agree with the myob, not worth it crowd. Any FA who is ranting about controversial subjects, in uniform, in a public space is just spoiling for a fight. You intervening is going to give them exactly what they want, and it's going to be 3+ of them against you and maybe your FO if you haven't been a jerk the whole trip. Not to mention the nearby passengers who are most definitely going to YouTube the ensuing screaming match. Getting written up for harassment and unprofessional conduct bypasses the CPO and goes directly to the snowflakes in HR. Who's side do you think they're going to believe? Then the CPO is instructed to administer your discipline.

Also, as said above, in this day and age, your Captain authority ends when you step off the plane. Regardless of how it should be, used to be, or your interpretation of the FOM/GOM description for CA authority. So yeah, if it's happening on the plane, you may have a leg to stand on, but you still intervene at your own peril. Off the plane? I absolutely wouldn't get involved. It's simply not your problem and not your place to be the karma police.