Franklin went with Val to Krakoa because he was curious to know it, he wasn´t kidnapped, all the X-men did here,(despite being written very OOC to force the traditional super hero battle) was to give Franklin an invitation to Krakoa.
This is one of those situations in which I wanted to get in the story but could not help but notice the spiteful way Dan Slott was writting the X-men, he simply didn´t like Krakoa´s take, wrote the X-men as if they were Kree seeking genetic purity and tried to make the FF4 sympathetic but this just made me dislike their holier than you actitude. I hate it when guest stars are written OOC just to make the protagonist look better in comparision, just because the writer doesn´t currently likes the characters or to force drama. The X-men adopted Franklin when Onslaught took his parents and were happy to give him back to them when they came back but acording to this they kindap kids le sigh