I cannot say in Bill's case what happened, but can tell you about a situation in my home town and the time frame is the same.
Back then, some small towns still had LE that would come by on parents request to get a child to tell the truth, say about stealing a piece of candy, and get the child to pay for the candy, and hopefully scare them away from a start in a criminal lifestyle.
Two ten year olds didn't come home one night for dinner on time on the street behind me. They didn't come home until after 9 p.m. When they got home they told their parents they had been kidnapped. Well, the parents called LE. LE came, parents and LE knew the children's story made no sense, nor would they be home a couple hours later if they were kidnapped. So, LE took them to the station for questioning and lie detectors tests. The children were scared enough to tell the truth. They had simply been playing in the woods behind the house and made up the lie about kidnapping to keep from getting into trouble.
In this situation, it may be the same. Bill's stories make no logical sense, and he is not giving any details, so they have to try to pull the truth out of him. They may have thought he was making it up to cover for something he himself was doing, alone or with someone else.
Not saying that is what happened, but it seems they didn't have a choice if he was refusing to give details or any sort of story that made logical sense. Being dry under a bush after being there for hours does not make logical sense, nor does being passed out for that long from being choked. So, with allegations of attacks, they had to try to force the truth out so they could find the attackers in order to charge them and keep any further harm to Bill or others from happening, and to get to the truth, in order to know how to proceed.
JMO