Well, I am certain I have no idea what it is Greg doesn't get about the power, but I will continue to speculate.
It's true that the power has two parts, only one of which has a "use", and that's changing the defensive player to being Siren.
The second part has no use, but is nevertheless still a valid part of the alien's ability, which is to gain a colony as a winning defensive player.
Now, on the bottom of the card, in the blue strip that indicates the capacity in which you can use the power, it reads "not defense", which corresponds to the "use" part of the alien. You can only play a cosmic zap when Siren is NOT THE DEFENSE, and is luring an attacker to Siren's system.
Siren is not the only power that has a use portion which is zappable, and a non-use portion, which is not zappable. Warrior, for instance, adds tokens without a "use".
Is Siren too powerful when it can't be zapped on a turn where Siren's color comes up in Destiny, and wins an encounter (thereby gaining a colony)? I'd argue no, but I'd also support the idea that you can still zap Siren at that moment, even if by doing so, it's considered a house-rule. Certainly something for the next FAQ to look at.
But in the end, I don't consider anything on the alien sheet to be a contradiction.
The original Eon version of Siren lured the offense when destiny was flipped, irrespective of what color came up (thus, even if it were Siren's system, it was still luring a player, and could win a colony). The Mayfair version clarified this in the description (and only restricted Siren from luring on a Wild Destiny).