Their letter is here, and it calls for excluding Wilders based on his harsh criticism of Islam and of Muslim immigration:
But beyond that, the representatives aren’t limiting themselves to relying on Kleindienst, either by name or by its logic; instead, they seem to be making a broader point about freedom of speech “[i]n the U.S.”:
In the U.S., freedom of speech is a bedrock principle that distinguishes free societies from ones living under oppressive regimes. Freedom of speech, however, is not absolute. It is limited by the legal and moral understanding that speech that causes the incitement of violence or prejudicial action against protected groups is wrong.
This is logic that applies just as much to Americans speaking in America as it is to Dutch lawmakers who want to come to America. In Rep. Ellison’s and Rep. Carson’s views, “freedom of speech” “is limited by” an exception for “speech that causes the incitement of violence or prejudicial action against protected groups.” And of course they aren’t just limiting their claim to the very narrow Brandenburg v. Ohio “incitement” exception to free speech, which is limited to speech intended to and likely to produce imminent lawless conduct — conduct in the coming hours or maybe few days (see Hess v. Indiana). Wilders’ statements don’t urge any imminent conduct (or even any criminal conduct, as opposed to long-term changes in the law). Such statements’ are “incitement” in the Congressmen’s opinion only because the Congressmen apparently view constitutionally unprotected “incitement” (or, as they term it earlier, “hate speech”) much more broadly.
Whether “Christian culture is superior to other cultures,” which groups should be allowed to immigrate into a country, and even whether Islam should be viewed as an ideology rather than a religion (an unsound distinction, in my view) are matters that the First Amendment allows us all to debate. The Congressmen quite clearly don’t want to allow Rep. Wilders to debate such matters here in the U.S. But their “In the U.S.” paragraph suggests that they view even such debates by Americans as constitutionally unprotected.
For more, see this Foreign Policy item.