Yahoo has him listed as TE because NO drafted him as TE. It's that simple. Yahoo won't fix his eligibility until next year, per their policy.
As for feeling guilty. As long as your league commissioner has no problem with it, or there are no rules against it. Play it. Chances are that he was a FA up until week 1 or 2 and was free for the first person to swoop him up. Those who whine because they slept on him can get over it, or try to trade for him. He's fair game.