The simple fact of the matter is that the organization believed that Sheahan was their young, second line center of the future. He played his way into that role over the likes of Jarnkrok who was "more talented", according to Jiri Fischer, which speaks to Jarnkroks work ethic. The fact ther was rumors of him wanting to bolt if he wasn't guaranteed an NHL spot further speak to that work ethic and being a team guy. So if Sheahan and Zetterberg are our top 2 centers for the next 5 seasons, which is conceivable, where does that put Jarnkrok? Either a 3rd/4th line center or on a wing and we have plenty of young Wingers that are just as good or better than him.
Seriously, we have had Zetterberg, Datsyuk, Weiss, Helm, Sheahan, Andersson, Emmerton and Glendening all play significant minutes for us at center this year. 8 guys that our organization thought were more NHL ready than Jarnkrok and all but Emmerton are likely to be here long term. The organization is still trying to push for a playoff spot, they realistically can still make the playoffs and Jarnkrok wasn't going to help them do that this year, and likely next year or the following. So he was considered expendable to get a guy in Legwand that can help us get in now and, if resigned, the next 2-3 seasons.
Keep in mind, Datsyuk is under contract for 2 more years. Zetterberg, Franzen and Kronwall aren't getting any younger, our best chance to win a cup over the next half decade is in this 2-3 year window before we have to start retooling again. Jarnkrok does very little to improve those chances of winning the cup in that time frame. If we resign Legwand for 2-3 years, he does improve those chances. After those 2-3 years we probably have a center in the farm that is just as capable as Jarnkrok was ready to come up.