In Athens, an "idiotes" was a citizen who focused only on private matters rather than participating in the polis (city-state). Because civic participation was considered a duty, this term carried a negative connotation of being socially irresponsible or uninvolved.
This term evolved into the modern "idiot" which we are familiar with.
Well wasnt that a good thing?
After the extermination of Melos they could credibly say they were less responsible for the actions of the polis.
And had a higher chance of deflecting the inevitable revenge on to the non idiotes Athenians.
And as a fellow Greek man said, "Just because you do not take an interest in politics, it does not mean politics won't take an interest in you".