it's to generate preorder sales. hence, p-2-w. doesn't really relate to purchasing the dlc content, per se. if that is a decision that you are going to be making, to buy the dlc regardless, then you would only be getting the weapon as a bonus (early access).
but the ways in which Bungie has included this as a "hey!! preorder the Iron Lords and get this unique looking skinned weapon!!" makes it sound as though they are grabbing for extra sales.
do you see how this works?
That's not pay 2 win though, pay 2 win is paying money for a game advantage if you don't pre-order you still get the new gally. a different color doesn't really seem like an advantage