Would you rag on CD players because they can't play gramophone records?
Actually yes, I would. Of course it won't be able to play a gramophone record. But what's the use of a CD player if it can't play back a musical piece that was originally recorded for replay on the gramophone? That doesn't make any damn sense. Every new technology should logially be downwardly compatible with older media forms.
In the same vein, why do I need the new HD technology, if it cannot display a show that wasn't meant for high definition viewing?
HD is backwards compatible, Blu-ray players can play normal DVDs and all HDTVs can upscale an SD image to display on a HDTV. And while an upscaled image is better than what you will see on an SDTV it is not as good as true HD.
For those who don't understand, here's a picture:
This is a very basic representation of the pixels on a TV or monitor, a HD screen has more pixels than an SD screen of the same size. As you can see, a HD screen gives a clearer representation of shapes but it requires more more data to store the image because (in this representation) it needs to store the data for four times the amount of pixels. A DVD does not have all this data, it only has the data of the SD image because that is all that DVDs were designed to carry, but a Blu-ray disc of another type of HD format does contain all of this data.
A DVD will work on a HD player, but it can't display in HD because the DVD does not contain the data to play in HD