User blog comment:Sky-Dragoon-Twilight/Weisslogia and Skiadrum, Alive or Dead?/@comment-7418318-20141208173058/@comment-9595671-20141209041510

Uh Nick, the reason they can use Dragon Force at will is because of the Lacrima they each possess. Natsu himself gave proof to this in Chapter 411 when he used it willingly despite it was only for a minute.

Their elements are the kind that relate to illusions after all, I've seen many series where someone with light and darkness powers or something similar to them use illusions.

We don't really know why they do the actions they do. Its very possible that they needed to do something different for Sting and Rogue due to the nature of their powers... or maybe something sudden made a change of plans, like what if they felt Acnologia nearby? If that was the case they would have needed to get their sons' to get out of the area and simply flying them away would have been a major risk as Acnologia could have spotted them.

Igneel did mention that they want humans to surpass them after all. If their slayers got killed that would have complicated things, it wouldn't be that easy to find new slayers, especially since time could have been running out before 7/7/X777 came.