Many Americans don't give a thought to what their heritage language is. No doubt this is partly because we are titans of assimilation and monolingualism (and I don't mean that entirely as a bad thing -- it's no doubt the reason why it's not ridiculous to speak of "Americans" in a country with dozens of ethnic groups spanning almost 4 million square miles). I'm only the third generation born in the U.S. on my maternal side, and I know exactly three phrases in German (excluding things I learned outside my family), and one of them is "Gesundheit". I think another reason we don't think about heritage languages is that we have so many of them. I don't know if I know a single second-generation (or greater) American who has ancestors from a single ethnic group. Depending on which line I'm tracing, my "heritage language" might be German or Scottish or Irish (or maybe even Italian or Hungarian -- old census and shipping records aren't the clearest).
In another sense, we all speaker our "heritage language", if we define heritage language to refer to the language of the culture we identify with. While the U.S. has no official language, English has become the de facto lingua franca, with 82% speaking it natively and up to 96% fluently. Insofar as English is the national language of the U.S., and insofar as I consider myself an American, English is in some sense my heritage language. But I think for many people that use the word "heritage", it means a lot more than this. It's not just about genes or cultural affiliation, but about self-identification. If a German child is adopted by Lakota parents and grows up speaking Lakota without knowing a word of German, is Lakota his heritage language any less than his parents? I don't doubt that there are many who would say "yes", but it's a touchy subject trying to foist heritage languages on others.
It's not by accident that I chose Lakota in the above example. The concept of a heritage language is of utmost importance for people who are losing their language. For native peoples of the Americas (I'm choosing North America because that's where I live and those are the languages I've studied the most), language is much more bound up in culture than for Europeans. European traditions, religions, and politics have been translated and adapted so many times that I have met very few people of European descent who identify strongly with the language they speak. While nuances are of course lost in translation, I would wager that few would say that the ideas in Machiavelli's The Prince would be lost if we lost the original Italian printing. On the other hand, there is a very strong feeling among American language speakers that losing their language means losing their culture, and means losing unique ways of looking at the world. (NB: there are other American language speakers who feel equally strongly for the opposite view.) Thus the concept of a heritage language is a very important one.
These are just musings. You may disagree with some or all of them. That's fine. One final thought: given that languages evolve, where does our "heritage language" begin and end? If English is John's heritage language, is Middle English? Old English? Proto-Indo-European?