Many people treat it that way.

If you're "Christianed" or "Baptised" CofE and Catholics will both mostly consider you Christian even if you're an atheist by confession.

The issue is that religious actions become cultural actions. And that religious leaders claim people as their own.

People who live in Thisistan are Chrewslims; or people whose parents were Chrewslims are also that. Silly.

Overall silly, but that history and location are important to understanding the ground in which a person grew, so to speak. There are some quips about "the God an atheist doesn't believe in" that have a point -- a person's worldview is shaped by ambient society and the philosophy they were exposed to as a child.

It is weird how culture and religion become decoupled and religious actions become cultural markers.