Why is the USA considers biracial people just black?

Yeah I wished they would let biracial be there own race but we do have a lot of false Biracial running around on Y/A claiming to be Mixed. Example someone who is 3/4 black will be claiming mixed when in fact there not mixed. When confronted about there race they wanna complain and say "it make me happy to be mixed with even a little bit of a different race" lol self hate is a b*tch.