No more "unproveable" than the statement: American High School graduates know what the First Amendment is." What's your point?
Since it's unproveable, it's foolish for the author to attempt to, at least in part, predicate an argument on it.
"The reason McCain Feingold is a stupid bill is that the First Amendment guarantees freedom of speech. Every high school graduate knows that, so why can't Congress figure it out?"
...would you then say my argument was invalid because I can't prove every single high school grad knows what the 1st Amendment guarantees?
There's a certain point where such nitpicking no longer serves the cause of reason, and I believe you passed that point a few miles back.