I'll tell you what they can do and what they should have done. If, on your iOS device you go to Privacy > Analytics you will find this: https://imgur.com/a/LTV9bGc
Where Apple explicitly asks you whether you are willing for your data to be used to improve a variety of services and you can opt-in or opt out. Apple dropped the ball by not specifically including Siri as a separate item in the list. Bad Apple.
So because some things are optional, everything has to be optional. Too bad that’s just not how it works. If you don’t want Apple to process the data you send to Siri your option is to turn off the service.
Here’s a little ‘secret’, you can’t use the location service either without Apple processing you data for analysis. If you don’t want that, your option is to turn off location services.
Where Apple explicitly asks you whether you are willing for your data to be used to improve a variety of services and you can opt-in or opt out. Apple dropped the ball by not specifically including Siri as a separate item in the list. Bad Apple.