The only thing I can think of, as to why Spitz and not the others, is that Spitz didn't see eye to eye with the others over the bash over the head being accidental.
But his words are hardly defamatory, he hasn't leveled an accusation against Burke, all he has said and demonstrated is that it was within the capabilities of a child, and his words "I cannot see anybody, even a child..." means it is not directed at Burke, but anyone taking a heavy object to a child's head.
Here's the extract I'm talking about (credit to PositiveLight for the transcript)
"Kolar- "My hypothesis was that I think the Ramsey's came home around 9:30-10:00. I think JonBenet
was asleep and I think John did carry her upstairs. Patsy remained downstairs with Burke and
served him with tea and the pineapple. I think that accounts for the physical evidence as well as the
latent prints. Then I think she got JonBenet up to make sure she used the toilet so she didn't
wet the bed that night. JonBenet was up, she may or may not have brushed her teeth, that stuff was
out on the counter and then I think she was up and awake enough that maybe was still hungry
and she went downstairs in the meantime Patsy continued packing for the Michigan trip. I think
Burke was upset about circumstances of Christmas presents he probably would have been upset
about her trying to snag a piece of pineapple, out of anger he may have struck her with that
flashlight."
Dr. Spitz- "I think we already agree on that."
Clemente- "Yes."
Fitzgerald- "Yes."
Laura- "Absolutely."
Dr. Lee- "Sure. Yeah, I agree with that. However I think it's not the intentional murder. "
Clemente- "I agree there's no evidence to support it."
Stan Burke- "So you don't think there was no malice of forethought. Something happened to cause that?"
Dr. Lee- "Spur of the moment or something like that."
Clemente- "Or accidental death. It doesn't have to necessarily be a murder."
Dr. Spitz- "How can it be an accidental death? I cannot see anybody, even a child, taking a heavy
object like a flashlight and hitting it on somebody's head as an accident."
Clemente- "Maybe in anger he swung it faster than he thought he would. We're talking about a
10 year old who by the way, who had hit the same person in the head with a golfclub. And what
happened?"
Dr. Spitz- "Nothing happened."
Clemente- "Nothing happened."