If a person bought RB2, downloaded the Free 20, and kept the game until the release of RB3, then they just use the code they already have to get the RB2 import. But from the very beginning, they were handing out codes for the import like they were candy to anybody who asked for one. And why not? You can't get the import without paying the extra $10 anyway.
An unused RB2 code is only strictly necessary for the Free 20. Beyond that, it only saves the hassle of getting a new one for the RB2 import.