yes, the DTV "invention" is limiting OTA station for each DMA; it's hardcoded in system tables and AM-21 do obey the tables regardless of real OTA signals;
dish took opposite approach - their OTA dongle [USB receiver] are free of any limit - you can get any station what you can get locally
Of course, the exception is the HR34. It will scan and show any available channel. For the channels not the the guide data (as shown in the OTA tables), the HR34 will simply say "Regular Programming" and won't have any detailed info.