Originally Posted by Sethonre
It's because, right at the end, he went to the Light Side. So he became "one with the force"
I think the argument is, why can Vader become one with Force automatically, when Obi and Yoda had to train for 19-23 years to achieve it.
It has been said that retaining one's consciousness in the force can only be achieved by lightsiders, but we know that Marka Ragnos was a Sith, and retained his, so I don't think light or dark matters.