In other words, he publicly harassed a colleague who (for what could be any number of perfectly valid reasons) preferred not to publicly state their beliefs. That would seem to me to be an eminently reasonable reason to fire someone. If you go around publicly harassing your colleagues to publicly state their political opinions, you deserve to be fired.
I'd like to portray another question for you to consider: Do you think being able to have this policy is an inherently privileged position? For the record, I don't disagree with you. I have the same one.
I don't want to assume anything about you, so I'll speak about myself: I'm a het cis white male. I'm well-educated and well paid. Politics basically don't affect me unless it's taxes (which is why our industry ends up leaning so heavily libertarian." I am able to CHOOSE when to discuss political/social issues because i am able to CHOOSE when they affect me.
This is not the case for many others, including I bet your coworkers. If you are a woman, non-hetero, non-cis, or a racial minority, you don't get to choose whether politics/society affects your life - it is automatic. There is no clean separation for a lot of people between work and life and it spills over, and even if they intend to not bring it up, it sometimes will.
I say all this not to get you to change your policy, but to keep in mind why others may not be able to have the same one. What will naturally follow from that, is that people invariably look for allies. So if someone asks you to discuss a subject, one that violates your policy, you should really consider whether your policy actually makes sense in the world, or if it only helps you while actively harming those around you.
So if you're an ally, you should consider flexing your policy, and trying to help.
And if you're not, well...then your silence IS complicit, and you shouldn't be surprised if it affects your career accordingly.