The only thing you've been "ripped off" of is time. The credits are a reward. Nothing was taken from you, except your time.
Wrong, plain and simple. The credits are payment for your time, your answers to their survey and for providing demographic information. They're not a reward. There is real money between WGT and the companies who put these surveys up on here, the only way people would do them is by being paid something, credits that can be used to purchase balls, or what have you here.
I don't really use surveys for credits that often, sometimes they work. Most of the time I just buy what I need. But when you do complete them it would be nice if there wasn't a hassle collecting on it.