I think it's absolutely valid to defederate if the users from a particular instance have a propensity to violate the content policy here.
For communication purposes, I think it's important to clarify and emphasize that this is the case and it's not done for political reasons. Maybe I'm overly paranoid, but I always feel as though this is a slippery slope (e.g. defederating from the socialist instance, or the trans/lgbt instance). Personally, for transparency's sake, I think it's better to avoid giving the impression that admins defederate from instances that they simply dislike. IMO, the defederation process should go by a process that is clearly defined and a published policies/rules.