referencing system catalogs

Can someone explain the reasoning behind not allowing
a user table to contain a foreign key of a system table?

For example:

create table myusers (
    login    text    references pg_catalog.pg_shadow(usename),
    ...
);

Fails with a message about using system catalogs.
Using pg_user fails because it is a view and not a table.

Is it because updates to the catalogs can be done outside
of transactions?  Is that a good enough reason?

thanks,

elein
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