When a typical large TV display cuts off inches from all edges of your desktop, I wouldn't classify that as a properly computer display. Even the most rudimentary PC display will always display edge to edge. Most TV's don't offer an option to turn off overscan or even acknowledge it's existence to the consumer. If they do offer adjustments anymore, they require a technical knowledge of the TV's program modes, which the typical consumer won't venture into let alone know exists.
On a PC monitor, one simply uses the supplied controls to adjust as desired.
They are not the same.
In all fairness, when HDTv's are used through the computer DVI input, they don't overscan. At least, not the ones I've used.
The point is really that an HDTV is in the living room, and the computer is usually somewhere else. An HDTV is a family, long distance viewing device, and a computer is a single user, close viewing device.
In addition, the color, contrast, and other viewing features of an HDTV is not intended for computer use, but for viewing consumer entertainment standards.
While I MUST see all the pixel detail from my monitor, I don't need to see all of it on my HDTV. That's another difference.