Assume you are playing on Deity/Sid, have set up a large number of (totally corrupt) towns somewhere on a distant continent in order to help with the unit upkeep, and suddenly the AI on that continent decides to attack with huge numbers. You see the stack approaching and have no means of defending your overseas settlements. Without a rule against this, you could now simply rush a settler in all these towns (you don't even care whether these towns have size 1 or 2, you just want to evacuate the population and get the hell out-a-there, before the monster stack arrives), put them on a ship and hope to find some other spot elsewhere, where you can re-settle them.
With a rule against this, you would either have to let the AI destroy all these towns, which would quickly net you a big war weariness, or destroy them yourself, before the AI gets there