Yes they do. This is indeed how supply and demand works, companies price what the market is willing to pay, it's just that in this case, the market is a clientele of 1.
Let's say we charge everyone the "big customer" rate.. The smaller customers couldn't afford it. If we charged everyone the "small customer" rate, we'd put ourselves out of business by spending far too time on the big customers while not making enough revenue to cover the costs of serving the larger customers. If we're practical about it, the smaller customers just don't have budgets as large as the larger customers. If we want the smaller customers to buy in, we have to reduce the price to something that fits their budget.
I just wanted to back GGGP's judgment of vendors who adjust pricing according to customer pocket depth. While I'm at it, your claim that individuals are markets unto themselves is also risible.