They're obviously getting OEM pricing like Specialized, Trek etc. You'll notice Ribble, PBK all sell complete bikes in addition to standalone group sets. My guess is the European regulatory environment prevents Shimano, Continental etc. from prohibiting retail sales of OEM parts. In the US I'm guessing no such regulations exist to prevent Shimano from preventing these sort of sales. Just a theory but it would explain why you can buy the same parts cheaper in Europe (weak dollar and all) with free shipping. Unless you believe they're selling them at a loss...

Also keep in mind that the consumer misses out on the warranty when they buy overseas.