Online product reviews have self-selection bias, yes, but it is not true that cables are reviewed only or even mostly by people who were not satisfied. For example, on Amazon we can see a 4.5 star mini USB cable with 679 reviews [1]. Similarly, Monoprice has a 5 star USB extension cable with 160 reviews.
[1] http://www.amazon.com/AmazonBasics-USB-Male-Mini-Cable/dp/B0...
[2] http://www.monoprice.com/Product?c_id=103&cp_id=10303&cs_id=...