On one hand, 'Buddhism' has always been and will always be influenced by the source of information for the students (specific teachers with their strengths and weaknesses, specific texts and translations with their strengths and weaknesses, etc), hence the 'lineages.' I don't have any difficulty to understand that new streams, new lineages, of Buddhism would appear in America. Buddhism has simply never been carbon-copy from one generation to the next!
On the other hand, I don't see how there's a collective in this, an 'American' version. Such 'collective' seems an illusion, there's very little agreement in the Buddhist community about the best way to realise मोक्ष, or even about the different possible ways to do so (that lack of agreement is not specific to America, I think "American Buddhism" doesn't make more sense than "Asian Buddhism:" all the main 'schools' are represented!). Moreover, 'American Buddhism' is often associated with the idea of getting rid of Eastern-specific traditions to 'adapt' the teachings to current mores and ideas in America. While adaptation naturally arises from varying conditions, I doubt it should come as a voluntary, forceful rejection of teachings considered 'unpleasant:' choosing to preemptively reject 'cultural elements' one doesn't like seems like falling into the very 'aversion/hatred' one should get free of...
The supposed adaptation to the 'singularity' or 'specificity' of the American people resonates with the idea of a collective American Self + denies interdependence + closes America to external influences; for what? Just because people cling to their American tastes and, for example, refuse bowing because they "don't want to bow to anybody or anything" and don't see how such a choice is definitely giving in to the ego?
Surely teachers will try and adapt to make the message the most accessible it can be, while trying to keep the 'essence' of it; surely, Dharma is impermanent, like the rest; but what is 'American' Buddhism?