Income is not collected on a non-island land territory unless it or at least 1 territory that borders it contains 1 friendly unit.
This adds a certain element of realism to the game.
I've thought about this as well. I looked at it from a slightly different perspective. All enemy controlled, but unoccupied territories are subject to a rate of reduced productivity 50% that of the original value rounded down ie. Captured enemy terr. with a value of 1 with no "police" unit present entitles the occupying force with 0 PU's (.5 rounded down), value 2 without presence = 1 PU, value 3 without presence (1.5 rounded down) = 1pu , 4 = 2 etc.