You've made an understandable mistake. You are probably looking at this picture:
That is not the unit circle, and it isn't even in the $z$ domain.
What you are looking at is the locations of the poles for a 4-pole Butterworth filter in the Laplace domain. These are values of $s$, not $z$, and the circle is not a unit circle -- it's radius is defined by the cutoff frequency of the filter (which is why the radius is indicated as being $\omega_0$).
The Butterworth is one of the "old modern" filters, invented before we could just start with a desired frequency-domain response and synthesize the optimal filter. All of these (Butterworth, Tchebychev, eliptic, Gaussian) were originally designed as continuous-time filters, and the canonical representations of them are in the Laplace ($s$) domain. Implementations of these as IIR filters in the $z$ domain are sometimes-useful approximations.