We are not sure who Thomas Parker is.
Perhaps AP thinks two wrongs make a right. In any event, the original quote was by Theodore Parker as we reported at the second link below.
From the next paragraph in the AP report
"It was not us that thought he said it, it was many people that believed - rightly so — that he said it," press secretary Robert Gibbs said.
If you said "I have a dream" does that mean from now on everyone should attribute that quote to you?
Back to the AP article:
Parker's adherents note the Transcendentalist and Unitarian minister wrote this in his 1853 treatise "Of Justice and the Conscience": "I do not pretend to understand the moral universe; the arc is a long one ... And from what I see I am sure it bends towards justice."
Jamie Stiehm, writing in The Washington Post on Saturday, said crediting King but not Parker "goes beyond the beige."