Why do people think all light skinned black people are biracial?

Im light skinned and im not mixed and their are alot of people in my city(cleveland) that are the same way... I can kind of tell the difference between a mixed person and a regular light skinned black person like my self..But i see alot of people on this web site saying Beyonce and Rihanna and Chris Brown are mixed. so just me being curious i did the research think they cant be mixed because i could tell if they were(although i was wrong about Hallie Berry who is 5 shades darker then me but is mixed) But in any case they were'int mixed at all just like i said.. Also Malcome X was really light and we know that he hated white people, and Huey P Newton was even Lighter and he founded the Black Panthers... IT DONT GET MORE BLACK THEN THAT!!!!