Because for every action there is an equal and opposite reaction, the stars in a binary system orbit around the common center of mass for the system.
This requires that Kepler's 3rd Law be modified to read,
where P is the orbital period, m1 and m2 are the respective masses, and R = d1 + d2. Notice the symmetry of this equation: since the masses are added on the left side and the distances are added on the right side, it doesn't matter which star is labeled with 1 and which star 2.