This rule is to avoid gamey tactics. It ensures a unit gets -1 if opportunity fire comes in, and -1 the next segment as well. The reason is so people wont avoid opportunity fire so that they can activate and fire the following segment to avoid the -1. As a previous poster mentioned, you should be able to remember what you moved a segment ago.
I thought the -1 on opportunity fire only applied to anti-tank fire. Surely with direct fire the opportunity taken blesses the firer with a + 1 ...or have I been playing it wrong all these months? No! I don't think I have.