On what grounds should Piers Morgan be deported?

Whether noncitizens may be deported for speech that is constitutionally protected from criminal punishment is not clear. But to my knowledge there’s no statutory authorization for such deportation; the government can’t just deport anyone it pleases (even when the person is not a permanent resident), but has to follow federal law on the subject. And even if Congress could authorize the deportation of people for speaking out in favor of various laws, it hasn’t done so at this point.

But beyond this — and even setting aside the question whether deporting someone for his speech should be seen as violating the First Amendment — the U.S. Constitution doesn’t just contain the Second Amendment. It also contains Article V, which specifically provides that the Constitution may be amended.

Even assuming that Morgan wants to argue for gun controls that would violate the Second Amendment as you think it should be properly understood, that’s not a “hostile attack against the U.S. Constitution,” any more than arguing for repeal of Prohibition in 1925 or the enactment of the income tax in 1910 was a “hostile attack against the U.S. Constitution. He’s not calling for violent revolution. He’s just arguing about what the right policy ought to be, and We the People have the ability to implement that policy, often without a constitutional amendment but sometimes with.