I think there are two things, one is what the actual law in Arizona says is rape, and the other is that there may be enough evidence to arrest him for a lesser offence to keep him off the streets while they investigate.
This being the USA there is also the possibility of a plea bargain, and although to us this seems simple - no consent = rape it is going to be difficult to prosecute.
The victim cannot give evidence, there is no physical evidence other than the child, no one knows the date of the rape or time so there are probably no witnesses.
If he claims she responded to him whilst he was carrying out personal care and he took that to be consent and that she orgasmed then it is down to the prosecution to prove her knew new this was rape.
In some places, and I think the UK is one, if you have 'genuine belief' that you are not commiting rape then you can use that as your defence.