To answer this thread's original question (in my opinion), it's because USB cables cost more money for Sony. And a lot of consumer electronics these days do not include the proper cables so that cable manufacturers can continue to rape the public with the outrageous prices of USB, component, HDMI, Ethernet, etc. cables. They figure that the public is already used to this disgusting trend, so it wouldn't be that big of a deal if they didn't include a USB cable with each and every controller when they've already given all PS3 owners one.
Or, it could probably be because Sony doesn't want us charging multiple controllers at the same time because the PS3 would overheat and blow up or something. I'm guessing it's the first reason I came up with.
With that said, yeah, it's pretty dumb that a $50 controller doesn't come with it's own USB cable. But is it really that big of a deal? Nope, not really. Like most people on this thread, I've already got a spare one from my digital camera (that I also use for my PSP). Also, if you've got enough money to drop on a PS3 and extra controllers, buying USB cables (if you REALLY need them that badly) shouldn't be a big problem.
This is yet another reason for people to bitch and whine though. That's what people on the internet seem to do best, especially when it comes to bashing PS3.