While I agree with your statement that "one individual's idea of 'anti-American' is another individual's idea of 'pro-American', the fact of the matter is that, in a lot of cases, one of those individuals can be proven to be wrong with relative ease.
Take, as an example, the Constitutional protection against compulsory self-incrimination. It's in the Constitution, it's woven into the fabric of all of our other laws, and it has been upheld countless times in courts at all levels and jurisdictions. I don't know how anyone could reasonably argue that a proposal that would violate that legal bulwark and tradition is not 'un-American'.
I think it's fair to say that, yes, there are controversies and disagreements where each side, however radically divergent one from the other, could probably make a good and reasonable case that their view is not, in fact, 'un-American' - scenarios, in other words, where neither side could justify applying that label to the other. Not all situations are that morally ambiguous - many are pretty straightforward.