While they aren't neccessarily box shaped any more, everyone knows what this phrase means.

(No cable in my home.)

this name brings back memories. this is what my dad called television. he was born in 1929 and spent his whole childhood and part of his adult years without the tv. we had one growing up and he watched at times. but he could not get why people were so attached to it. he could easily go outside or do other things that watch "that idiot box". it does seem like the tv can have some totally idiotic shows especially some of those reality shows.

No need to go into further detail on this. It's very self explanatory.

As in "A box that idiots stare at?" Perfect description.

Well, I don't know how much the word "box" applies to the many who use flatscreens (perhaps a little since they still retain the same basic shape), but there is still the matter of the programming displayed on its monitor, and quite a bit of it fits the description of "idiotic". Therefore, sometimes the word "idiot box" is quite a suitable name.