I used Vatican because it was one of the Pontifical Councils, in consultation with the CDF, as well as a further clarifying missive from Benedict XVI that clearly stated that ones membership in the church continued inspite of any act to the contrary (albeit with certain penalties depending on the circumstances). I did that to clarify their action from that of the CCCB, who issued the message referred to in the OP.
While you claim that many renounce their US citizenship informally, such isn’t the case. There were a number of individuals who sought to avoid their obligations during the Vietnam war by casually “renouncing” their US citizenship in Canada. The only way to cease being a citizen is to formally renounce that citizenship and have that renunciation accepted. US citizens have a method to do that, Canadian citizens have a method to do that, as do most citizens of countries around the world.
The Vatican states that with regard to
membership in the Catholic church, there is no such renunciation possible. Much like the Eagles song “Hotel California”
Yet, while insisting that people can never actually stop being Catholics, the is a desire to tell these people that they shouldn’t call themselves Catholics because they don’t believe the same things as other Catholics. So it’s quite simple,
if there are people that the hierarchy would rather not have referred to as Catholics, then stop insisting they will always be Catholics.
Otherwise, if the hierarchy wishes to continue to insist that these people will always be Catholics, it should get used to a future where it will have to share the name Catholic with groups of differing beliefs. It’s quite simple, really.