To say Jon has no authority to do that is again to ignore that neutrality cuts both ways. Why should Jon be neutral if no one else is? When a lord or house skimps on their duty to help out the NW, the NW should be allowed to support another lord who will do that.
Alys Karstark, the rightful heir to Karhold (or at least a representative of the rightful heir) came to the NW of her own accord asking for the NW's help. Cregan Karstark went there to demand her back so he could forcibly marry and kill her. Why is Jon bound to help Cregan, which in its own way is a breaking of the neutrality vows of the NW, when Alys is the one who came to ask for help, when Alys is the rightful heir to Karhold, and when Alys could potentially help out the NW in the future, while Cregan Karstark is none of those things and will not help out the NW in the future?
I for one see the Karstark/Thenn alliance as an important plot point, in so far as it being representative of Jon's attempts to integrate the wildlings back into Northern society, which I imagine is very important for a cultural area that has lost so many people in battle and needs to be fortified and unified in order to defend against the invasion of the Others.
And yes, while I love the Boltons (well Roose at least for being ridiculously entertaining), it would make no sense for a Stark in Winterfell to allow the very same combination of houses (Bolton and Freys who essentially tried to annihilate them from existence) bloodlines to live on.
Edited by Tagganaro, 23 July 2012 - 01:14 PM.