Why else would Salim use the word "gunfighter," an Americanism coined in 1890-1895? This word didn't exist during the actual cowboys 'n' Indians era shown in countless Westerns. It came into existence only when Americans began bemoaning the end of the frontier and fantasizing about the past.
For more on the subject, see America's Cultural Roots.
Incidentally, I'd say Slumdog Millionaire deserved the Oscars it got, including Best Picture. Rob's rating: 8.5 of 10.
P.S. Yes, I know the Indians in Slumdog Millionaire are the wrong kind of Indians. Feel free to make all the jokes and puns you want. <g>