Correct. And that is why the software is not downloadable. Apple sold several models of computers without N capabilities, but the hardware could do it. They also didn't advertise this feature, nor did they put the sales of the computers on their books as a "subscription with deferred income". Apple then released a firmware upgrade to add in the N capabilities, but had to charge for it due to Sarbanes Oxley. The Airport Extreme N comes with the enabler in the software, thus falls under the same rules that it needs to be sold.

Same reason they had to charge something for the iPod touch update recently.