Morality and Politics are not purely aesthetic.

Some fundamental aspirations are aesthetic, but means of going about achieving those ends are amenable to scientific comparison and some means can be objectively more or less effective than others.

Most moral and political convictions people express are not such fundamental aspirations, but derivative in nature,  expressing implicitly some notion of the best means of achieving some other end.  This translation of desires to actionable imperatives is very amenable to logic, reason, and the scientific method.

Moreover, since political and moral principles produce actions, it is possible to inquire as to the effects of such principles upon human communities and to justify that certain sets of principles are, objectively, self-contradictory, or objectively harmful within a given notion of utility.