They don't have to keep them banned. In fact, if they only banned them when a chip was detected and not otherwise, this would hose the people who bought chipped boxes to begin with (who MS should be focusing their efforts against anyway), because they wouldn't know how to remove their chips.

MS is permanently screwing over everyone with a chip just because they can. They don't have to. This is going to have some seriously whacked repercussions in the secondhand market, as you won't know if a box you buy has already been banned or not.