And the internment of American citizens with Japanese heritage was also found to be permissible. That doesn't mean that it was Constitutional by any reasonable standard, it means that our horribly flawed system doesn't always function correctly.
>Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press;
That is the text of the Constitution. It is simply absurd to suggest that forcing a member of the press to disclose their source does not "prohibit" much less "abridge" the freedom of the press. The founders understood that the independence of the 4th estate was absolutely critical to the function of a self-governed republic. Allowing the powerful to pass laws criminalizing journalism by forcing them to reveal their sources under penalty of imprisonment is simply an end-run around the Constitution by those who wish to retain their power and remain beyond scrutiny. This is especially true in the current environment, where the government classifies virtually everything they do, down to the most mundane government functions. Democracy dies in darkness, and so do republics.