“de” is not in that sentence a preposition but an article required for “Américains” like one would be in:

• “Pour les Américains, …“
• “Pour des Américains, …“

Note that in the similar

• “Pour nombre d‘Américains, …“

“d” is not an article but an elided preposition “de“:

• “For a large number of Americans”

Note that the article used is different depending on whether the adjective precedes the noun or follows it:

(1) De nombreux Américains considèrent que …

(2) Des Américains influents considèrent que …