Actually, all French territories are treated the same. French Indo-China does not get any special treatment other than that Japan is nearby and can take it.
In order for an Allied power to take control of a French territory, an Axis power must first take control. If you move into a French territory with noncombat, you do not gain control or add the IPC value to your income. You may only seize a French territory if the Axis take it first. The same is true of all powers who have lost their capital. For example, if Rome falls, Germany cannot just walk into Northern Italy and take it. However, if the Allies conquer an Italian territory (and Rome has fallen) Germany can take it and use its factory, build facilities, or collect IPCs from it.
If you were to liberate Paris, all territories which the Allies had seized from France (because an Axis power overran them and then the Allies counterattacked) return to France.