Rickard Karstark and the Night's watch

[Balerion06]

If Karstark was held prisoner until the war was over before being sent to the Wall, in say Riverrun, then some loyal to him might still try to break him out (and there could easily be other Northern lords who want to see him free beyond those from Karhold). If he does escape, he becomes a powerful rallying point for Robb`s dissenters (especially the Karstark men) and becomes a major liability to Robb`s war effort.

If he`s sent to the Wall immediately (ignoring the fact that the Ironborn hold major territories in the North), then Robb will be only wasting men getting him an escort all the way up to the Wall. Said escort will also not survive a very probable ambush by Karstark loyalists, and again we get a scenario of Karstark going free.

Finally, if Karstark himself did not ask for the option to be sent to the Wall, then Robb looks weak for suggesting it, especially because he desperately needs to set an example in front of his increasingly dissenting men,

[Jon Wolfwalker]

Robb sending him to the wall would have created an enemy. Not all brothers mean their vows(Janos). Robb wasn't that naïve where enemies were concerned. His fault was in trusting the allies he had.