This isn't an unlimited loophole in the constitution (at least not so far).

As the article you link to points out, "It is not always clear when the government is speaking for itself instead of unconstitutionally restricting others’ speech...The Supreme Court has not yet provided a clear standard for this type of case."

Also, "...even though government speech is not regulated by the Free Speech Clause, it is still subject to the Establishment Clause."