People don't always engage in good faith. Such people are not bringing ideas to the marketplace, they are trying to manipulate people.
In order to really engage with each other, we have to have some common ground on which we can work from. If that base ground is not established, there is no discussion to be had. If I'm trying to talk about how to make grocery stores more efficient, but you're talking about how to get to Jupiter, we can't have a conversation that has any point.
A similar thing can happen at the instance scale.
Defederating for the reasons you said are, by themselves, poor reasons I agree. But sometimes I think they are trying to say they aren't engaging in good faith, or that enough of the basic point of that instance is at odds with the basic point of this instance that defederating makes sense.