Soo I want my Camera Roll (CR) to be a "to save to computer" list, and leave things on Photostream (PS) until they expire, to share among my devices. Apple doesn't like that. Because PS has unlimited storage, they don't want to give it to you for free. To be on PS, it has to remain on the CR of the device where the photo was originally created. Deleting it from CR forces you to remove it from PS. Unless you do this.
If you connect an iPhone to a Windows computer, it shows up as a camera device, a bit like USB Mass Storage, but with limited functionality. Anyway, it lets you delete things from the iPhone using Windows Explorer. This removes them from CR. But leaves them on PS. That's it. DELETE YOUR PHOTOS USING WINDOWS EXPLORER AND THEY'LL DISAPPEAR FROM CAMERA ROLL, BUT REMAIN ON PHOTOSTREAM. WARNING this directly deletes your photos, they don't go to the recently deleted album.
Update 15 Apr 16
I found another method, since, you know, Apple doesn't like people using PS and they could "fix" this at any time. Use the app Google Photos (hoo boy, Picasa Web Albums, Google+ and Google Photos is another mess). Open a photo, tap the 3 dots in the top right, then "Delete device copy". This deletes from CR but leaves the PS copy PLUS it appears in the Recently Deleted, if you make a mistake. Something deleting using Windows doesn't do.
While I'm at it, what deleting/copying things from where does:
- Delete from CR - removes from CR, puts in Recently Deleted. Removes from PS, EVEN WITHOUT WIFI
- Delete from PS - leaves in CR
- Save from PS to CR - doesn't create a second copy in PS. But if you delete the copy in CR, it deletes the original in PS too