Same reason it's called a foul line is my best guess. Lets you know if a ball is fair or foul. I've actually heard it called a fair pole several times.
Its called a foul pole because it is an extension of and functions identically to the foul line which is also considered fair territory. Now that that mystery is solved the next logical question is why do they call the white stripes the 'foul lines'. Nothing magic there. Logically that is just what won out in the arbitrary process of naming things. Just like everything else such as bases, plates, bats, on-deck circle, dug-out, etc. that could have been called something else.