The Dropbox "Camera Uploads" feature doesn't work that way. What it does is, when you take a photo, it copies the file to the "Camera Uploads" folder in Dropbox, which then syncs to your Dropbox account like any other file.
It's a way to backup your photos, so that if something were to happen to the photo on your phone you'd still have the Dropbox copy. Or, more likely, you can confidently clear the photos on your phone (maybe because you're low on storage) knowing that you have a copy of it safely in the cloud.
There's no way to automatically remove a photo from "Camera Uploads" if deleted from your camera roll. (This is something that's bitten me once or twice. I use Dropbox, Google Photos, and Facebook to automatically grab my photos. It makes it a bit of a challenge to get rid of those photos of the inside of a pocket.)