Obviously Spike Lee can support whoever he wants for president, or no one at all. However, Bernie Sanders seems to be the candidate with the most similar views to Lee out of all of the candidates, so it seems like he would want to support him. Sanders marched with Martin Luther King Jr. and seems genuinely concerned about racism and other issues that Lee seems to care about.
Spike Lee might be famous for directing and producing movies, but he's an American citizen who doesn't work in politics. As such, he's entitled to express his political opinions freely. Whether fellow voters should base their decisions on the endorsements of film directors and other celebrities is a completely different question.
Spike Lee should support Bernie Sanders because his proposals would do the most to alleviate financial inequalities that plague the black community compared to other ethnicity's. Compared to his Democratic and Republican rivals, Sanders is the most committed to private prison reform and shrinking the gap between the nations top earners and the workers who grind through the business days to make the company money.
Spike Lee is correct in his support for Sanders because Sanders has been a civil rights advocate since being a young man in Brooklyn. He participated in the March on Washington and has even been arrested for fighting for equal rights for African-Americans. Spike Lee knows that Sanders has supported racial equality since the 1960s.