I think this is what makes the cut so hard. No one is an island, even the most toxic person. If you cut them off, there very likely will be people close to them of whom you are fond, but through sheer practicality, have to reduce contact with them as well. It's easy (relatively only) to cut off a person who is overtly damaging or dangerous. It's much harder to do so when you know that it means you won't get to see their spouse, or child, or whoever as well.
It sucks for the person who ends up as collateral. These things happen and it's not personal. It should, if anything, be seen as a symptom of the toxic person in question. "Alice is so toxic to Bob that she's made it not feasible for him to speak to me anymore." It's not always very clear-cut, and the cleverer toxic people are often good at hiding the damage they do to others so a cut might seem, to an outsider, unfair. Being collateral to a cut can then feel very unpleasant. But it should always be remembered that it's not personal, and is the result of another person.
The cut, after all, is done as a last resort, when the damage done by a person is so great that all attempts to temper it have failed and it is not possible to suffer it any longer without coming to harm (or more harm, depending on the situation). It's not much of a "choice" in the normal sense. It's a forced hand. If someone is beating you over the head with a club, sitting and taking it is certainly a choice, but it's not one that any rational person would make. It's important to remember that, I think. The cut is not done out of spite or desire, but sheer practicality and self-preservation. It's unpleasant to be affected indirectly by that, but one shouldn't begrudge the person who was forced to do it. The only person who made a voluntary choice was the person who forced their hand. It's tragic that people like this put others in the position where they are forced to cut off otherwise good people.