I'm fairly sure it's not legal, because you're acting in bad faith (while you haven't changed the License terms, you are effectively coercing people into not be

I'm fairly sure it's not legal, because you're acting in bad faith (while you haven't changed the License terms, you are effectively coercing people into not being able to take advantage of the license). I'd also consider the tactic immoral because you're adding restrictions to free (and copylefted) software.