I thought this was common knowledge now.
It's not just HDMI and home theater cables too. Here is a 6' USB 2.0 cable from Best Buy for $24.99:
And here's basically the same cable at Monoprice for $1.30:
Never underestimate good marketing + gullible customers.
On top of in almost every store when I balked at the prices of $30-70 the salesman would try to tell me that if I bought a cheaper cable it would be subject to interference and would not work well. Most people who aren't technical and don't know that's bullshit don't stand a chance. They listen to anyone they feel is an authority on the matter. "Surely the guy at Futureshop or The Source knows more about tech than I do, I should listen to him. I can't find a cheaper cable anyway."
The margins on TVs and such are apparently pretty low and the way that they make money is on the high margin accessories they push on you. They will give you a "deal" on the cables so it looks like it's working out in your favour but it's still a scam. Hook, line, and sinker. Because it's only $5[-100 over the price of the set, which is likely over $1000, most people will just go with it because it seems like a small price to pay to get the most out of their shiny new TV.
Quite often, we make more money from people buying a $40 cable than we do from people buying a $2000 iMac.
I also love how many Apple accessories the stores around my area have, simply because they make more selling cases and protectors than the actual device.
There is actually only one just Apple store in my town, and they have a radio commercial in which all they say is that they sell iPod accessories. It seems almost sad that it is the only thing the store is trying to sell.