This is actually standard ancient Greek rules of accentuation. It's due to the τε being enclitic. Enclitics are so called because they "hang on" to the previous word, and thus, according to some rules (which Smyth has outlined), they change the accentuation of the previous word.
For δῆμόν, it's because it's properispomenon, i.e. its penultimate syllable has a circumflex accent:
c. A proparoxytone or properispomenon receives, as an additional accent, the acute on the ultima: ἄνθρωπός τις, ἄνθρωποί τινες, ἤκουσά τινων; σῶσόν με, παῖδές τινες.
But πόλιν, on the other hand, is paroxytone, and thus it stays the same, while the following τε loses its accent:
d. A paroxytone receives no additional accent: a monosyllabic enclitic loses its accent (χώρα τις, φίλος μου), a dissyllabic enclitic retains its accent (χώρας τινός, φίλοι τινές) except when its final vowel is elided (174 a).
Note, though, that this change also happens in Attic. The dictionary form of the word is δῆμος in both Homeric and Attic Greek, and in both dialects the accent could change from an enclitic.