But what has her state of wakefulness got to do with her murder? If they were the killers, why would they lie about something like this? First rule of lying -- tell the truth as much as you can. So you lie about her being awake, then you have to also lie about her eating the pineapple. Why not just say she ate some pineapple and went to bed? If the pineapple has nothing to do with the crime (wasn't drugged etc), there is no reason not to say she ate it. Simple really!
For their purposes, they want it believed that JB fell asleep in the car, was carried into the house, and they never saw her alive again. They had no idea the pineapple would show up in an autopsy, and frankly, I don't think they thought much about an autopsy at all at the time. For their purposes, they also wanted it believed that BR never woke up that morning; later JR admitted BR was awake but they felt it was better to "keep him out of it" by saying he was asleep the whole time.
Eating the pineapple after they arrived home makes her awake at that time. Her "wakefulness" means they lied about her being asleep (though their son said his sister was awake when they got home). It also places her time of death pretty close to the time they arrived home, only two hours at most. Figuring that most children go to bed before their parents (and they already said that BR stayed up a bit later with his father playing with a toy), let's say for argument's sake that they arrived home at 10 (they said this themselves). Then possibly BR went to bed around 10:30, followed by his parents around 11 (if they went to bed at all is another matter, I am just theorizing a timeline). Possibly Patsy was awake even later, as we all know moms have a lot of little details to finish up before a family trip. With JB eating a pineapple snack (maybe BR did too, for all we know) a little after 10, that places her TOD exactly when it would be to allow for a 2-hour digestion. And (here's the REAL reason to lie about the pineapple) that places her death at the hands of intruders MUCH too close to the time that her parents would very likely still be awake in the house, even if they were in their bedroom. Her scream around midnight places her death very close to that scream, and that also happens to correspond to where the pineapple would be if she ate it when they got home and died after the scream.
As you said, why lie about it at all? I agree- it was an innocent enough event, whether the intruder theory is true or not. So why lie? If an intruder fed her the pineapple (and remember Patsy's prints were on the bowl, no stranger prints- the bowl was not wiped like the flashlight was) then she was eating the pineapple with an intruder in the dining area while her father and brother were playing with a toy in the living room and her mother was getting stuff ready for the trip. THAT is why they had to lie. The timeline just doesn't add up. The finding of that pineapple at autopsy stunned them. They never even thought to put away the bowl. They simply didn't think it was an issue. It is because they lied that I feel they are strongly implicated in the events of that night.