The title is deliberately provocative. A big part of the problem is that a term like "propositionalist" is not well-defined in debates about politics. Propositionalism is supposed to be bad, and it's supposed to have something to do with believing that the essence of being American is believing certain propositions. Hence, supposedly, it involves ignoring things like the love of a place and culture.
But that's not really a definition. How much love of place is necessary not to be a propositionalist? What is meant by "culture," and doesn't "culture" have something to do with beliefs? (If someone is a Christian, that involves his beliefs and will necessarily influence his cultural practices, like praying to Jesus and going to church.)
So after the manner of boring analytic philosophers the world over, I propose that we disambiguate the term, separating it into propositionalism1 and propositionalism2. I say that propositionalism1 is naive and foolish while propositionalism2 is sensible and importantly right. Here we go:
Propositionalism1 includes, but may not be limited to, the following theses:
1. All groups of people the world over, regardless of ethnicity or cultural background, are equally likely (or approximately equally likely) to produce good naturalized American citizens.
2. All that is necessary to turn people into good naturalized American citizens is to teach them certain propositions about the nature of politics, freedom, and the like, which they will eagerly embrace as soon as they understand them.
3. It would be wrong to discriminate in any way in immigration policies on the basis of ethnic or cultural background.
4. The success of the American form of government can readily be repeated in other countries with vastly different cultural backgrounds from America's.
Propositionalism2 includes, but may not be limited to, the following theses.
1. America has become great in no small measure because of the nature of the form of government put together by America's founders, who were right in their propositional ideas concerning the wisdom of the details of their system--e.g., checks and balances, separation of powers, freedom of religion, and the limitation of federal powers to those enumerated.
2. There is another set of important ideas presupposed by the American form of government which are not, unfortunately, exemplified in all other countries of the world and are centrally important to America's greatness. These include the evil of government corruption, the equality of persons under the law, the value of honesty and hard work, and the importance of the rule of law.
3. An understanding and love of the ideas in #1 and #2 is a crucial part of being a good American citizen.
4. It is not only theoretically possible but also a live, practical possibility that some people not born in America will develop this understanding.
5. If people can come to embrace these ideas, there is a good chance that they will make good naturalized American citizens. In fact, Americans born in America from generations of American citizens who scorn these ideas may be worse and less loyal citizens than those naturalized who have a deep understanding and love for these ideas. Those who have no concept of these ideas have suffered from a sad gap in their American civics education which should be remedied if and when at all possible.
6. Being of non-Caucasian lineage is not by itself sufficient to make it so highly unlikely that one will embrace these ideas and become a good citizen that all persons of non-Caucasian lineage should be debarred from coming to the United States and attempting to become citizens. While race and ethnicity are closely bound up together and can be important cultural markers, race by itself is not everything and does not automatically designate cultural fitness or unfitness for presence in the United States and future good citizenship.
7. Loving one's soil and kindred is not enough to make one a good American, per se, as opposed to a patriotic citizen of some country (any country) or other.
All of this is likely to ruffle some feathers. I'm happy to consider myself a propositionalist2, and I don't think this should be mistaken for being a propositionalist1.