Your not really forced to buy a HD. The madate was to do away with analog TV (NTSC) and go digital. Soon all new TV's will be capable of receiving digital broadcasts.
For TVs, here is an excerpt from the
Wikipedia entry:
As of March 2006, all 25 inch and larger TVs for sale must have ATSC tuners capable of receiving the 8VSB modulation used for free terrestrial digital broadcasts in the United States. The final conversion step is a mandate that all televisions and TV-tuning devices have ATSC tuners by March 2007. Many of these will not be capable of displaying HDTV signals at their full resolution, but they will be capable of decoding and displaying these signals.