Why do most of the Hollywood movies have at least one non-white guy?
Are there any casting guidelines for that or Just a coincidence?
Well first of all I'm not sure the premise of your question is true. I can think of several movies off hand with just white actors in significant speaking roles.
If you reduce it to 'a majority of modern Hollywood movies' then your observations might be closer to the mark, but I don't think this is down to any form of explicit policy of positive discrimination.
The USA is an ethnically diverse country. Ethnicity is a difficult area to quantify, but from recent census data people who identify themselves as white form just over 70% of the population (Wikipedia), so if a movie is to represent the normal makeup of the population then it would normally have a significant number of non-white actors. The percentage of white population decreases in urban areas, where (I am guessing) a larger proportion of movies are set. I am sure the casting team take this into account, both to simply represent normality and to maximize the marketing potential of the movie.