Charging for Loyalty

Seems counter intuitive, but successes like Amazon Prime seem to point otherwise. What is the value tradoff?   In CustomerThink.   " ...  Let’s make one point clear: Fee-based rewards are not pet rocks. Sure, some major merchants have recently launched fee-based loyalty initiatives, but that does not make the concept a fad. Rather, a retailer’s decision on whether to apply a fee to its rewards membership – or not – is very strategic in design. .. "