Mainly it's their deceptive advertising practices. Instead of paying to sponsor forums they make fake accounts praising their service in a random thread, and a second random account will jump in seconding the praise. The accounts will have been made that day, and have no previous posts. They spam random forums like this, and it's not considered proper etiquette, especially when perfectly good sponsors pay to be on the forums. Another reason some people complain (although I cannot independently confirm this one), is that when there is a shipping or order issue, their CS team can be slow to respond or unresponsive.