Errr... because I don't live in the US!
They're generally around £40 here, I mean if anyone can find the official one from a cheaper, trust worthy shop, I'm all for it but I can't.
It's more like a £25+ saving, some costing only £11
Even if they only have around have the capacity of the official battery, as long as they don't do any damage to the camera, I think that's a great price.