If you search for "United States inventors" you get the list you were probably expecting.
Google seems to like to include results appropriate for common phrases if you include only part of the phrases. My guess is that the "american" in "american inventors" brings in results for "african american inventors" too.
Imagine you were searching for 'american basketball player' and the selected individuals were predominately white. And on top of this you had guys like let's say Mike Dunleavy Jr. placed ahead of Michael Jordan for no apparent reason. Do you think this would be appropriate?
Google seems to like to include results appropriate for common phrases if you include only part of the phrases. My guess is that the "american" in "american inventors" brings in results for "african american inventors" too.