The Study of Creating Unrest in Settlements.

[Conqueror]

If the settlement has a different culture type than the faction that controls it, creating unrest with spies will be much easier. You can also use assassins to sabotage the PO-boosting buildings, which can cause massive unrest, although the AI is quick to repair the damage.

[PershsNhpios]

Thank you, this obviously explains why it is impossible for Roma to rebel, and why Arretium and Co. are likewise.

However, Taras rebelled without problem, and Bononia and Segesta - no closer to Rome, did no such thing - but lowering the taxes certainly makes sense.
I under-estimated the AI.

Well, this will be good if my war goes foul and they take Mediolanum - and certainly it will be of use in tampering with faraway factions who are doing well and have distant provinces.
(It will go well with my tactic of feeding their poor neighbours mnai).

However, it is agreeable then that one cannot use spies to assault the homeland.

Pity, now those short little poets are going to be rather dead, when they could have simply been expelled.

Thank you all for your answers.