This is probably a very stupid question. In many places (e.g. here), the Butterworth filters, e.g. lowpass, are described as being "allpole" filters, that have all of these poles on the unit circle.
But shouldn't poles on the unit circle lead to a pole in the amplitude response of the filter at that particular frequency, in the same way as a zero attenuates that particular frequency? Instead, the Butterworth lowpass has strong attenuation in the upper frequency spectrum, despite having all of its poles on the left-hand side of the unit circle (the high frequency side).
Could anyone please clear my confusion ?