Because Fan of Knives does not utilize combo-points.
We'd love to get rogue AE higher if we can do it where Fan is only used for AE and not as a general part of the single-target rotation.
Also note that comparing anyone to a Ret paladin recently, and especially at 70, isn't going to be a great comparison. Elemental isn't designed to have an advantage over mage single-target dps, but rogues should have an advantage over warrior single-target dps.
Keep the Energy Cost at 50, but remove the cool-down completely. We can use it 2 times in a row, but we will be energy starved. It won't do a ton of damage with daggers, and it's mediocre for swords. Any Rogue who wishes to spam fan of knives to get other rogues out of stealth will be much less able to attack due to the lack of energy. It's true that if a rogue vanishes, Fan of Knives will be the most likely used moved next during such a time to bring the rogue out of stealth, but any other class with an AoE already does this.
I see no problem with giving Fan of Knives free of cool-down, especially since it offers no combo-points or utilizes them in any way. It can't be used combat potency for combat, and if it really bothers blizzard that much, remove it from focused attacks as well.