The word “politics” is vague, and that only makes banning political discussions worse if it only becomes political when the higher-ups don’t like it.

Say your company has a possibility of working with some client company who is directly or indirectly involved with cause X. If it is “political” to talk about not working with them because of X, but it is “not political” to talk about working with them, then you see what I mean.

It doesn’t have to be a destructive conversation: one employee might say we should avoid them, but you might say we need to work with them because we need the money now and can drop them later when we are in a better place. Other employees could talk how cause X is not that unethical for reasons. If someone balks at a point of view incompatible with theirs and is incapable of expressing a viewpoint in a way that respects other views, maybe that someone is not mature enough and next time your HR can avoid that type.

Many people that ban political discussions miss the irony that it's a political decision.

Yeah exactly, the same people who shout the loudest about "everything is politics" and want to talk about it at work would go apeshit if someone at work said "I'm not comfortable with abortion", etc. HR would quickly be called and shut them down.