Umm actually there is because Apple invented the Dock Connector and the Dock Connector port. It is not a simple USB charging cable because there are all sorts of things to take into account. For example pin layouts according to the type of device is being hooked up to.
For example I have the Apple media cable which has USB and RCAs to allow me to hook up to my TV. There has to be pin layouts to compensate for the USB as well as the video signals. These are not standard USB. Only 4 pins of the 30 are for USB.
Apple has changed the Dock Connector at least once before when they moved the power to different pins to compensate for HD video being sent out the Dock Connector. Therefore the cheap cables aren't necessarily going to work. I have a Belkin cigarette lighter adapter for my car that won't work on my iPhone because it was designed for the old Classic iPods.
I prefer to buy Apple products because I know for 100% certainty that they will work and be guaranteed to work with my Apple products. To me that is worth far more than saving a few dollars on a cheap product. Now if it's something that I need that Apple doesn't make (cigarette adapter for car for example) I will buy third party but only the brands I know and trust which are generally not that much cheaper than Apple.
You want to cheap out then all you'll get are cheap products and that's fine if you don't mind that but quality costs.
But I agree with the posters saying Apple is only going after those using the "Made For iPod" labels when they are not part of the program. Essentially they would had to have reverse engineered the Dock Connector to get pin layouts and that means you have no guarantee that they got it right. This will ALWAYS come back on Apple not the product manufacturer so Apple has to go after these cowboys just to distance themselves.