The last time I bought an HDMI cable I checked local stores because I didn't want to pay shipping on a $5-10 cable. The cheapest cable I found was on sale for $20 or something. Turns out it's cheaper to go with Monoprice and pay shipping on a $4 cable than to get one on sale in real life.
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.