Seems a little curious to me too.. I mean you can go to BB or CC and get an HR21 for $199, so to me you'd have to be no higher than $99, even then, one would seriously have to think about whether or not they were going to need HD in the foreseeable future (like < 3years). But then to that, if the unit is upgradeable, what is to stop people from buying it at my assumed $99 and then "upgrading" it to HD?
And if it is the same platform and hardware restricted by software, why maintain the extra model and deal with supply, troubleshooting issues?
Assuming it is upgradeable, nothing is stopping a user from doing what you describe. That being said, nothing stopping DirecTv from charging a HD enabling fee either.