The rule to which you refer is actually "All electronics larger than a cell phone must be removed from the bag and placed in a bin in a single layer". Technically speaking, a digital camera is electronics, and an argument can be made that a DSLR body is larger than a cell phone.

The rule was a response to people who are now commonly traveling with both laptops and tablets, as well as their phones, and power banks, and chargers, and all sorts of other stuff. Layering these things together in your bag makes it very difficult for the TSO to see what's in your bag with the x-ray scanner; the images of the electronics all overlay each other and get confusing. So the rule was put in place so that electronics will pass through the scanner in a single layer, making them easier for the TSO to interpret.

I don't travel with my DSLR any more, I've switched to a smaller superzoom bridge camera, but I have not been required to pull any camera gear out of my carry-on. I think that's for two reasons - 1) the rule is mostly to prevent overlapping tablets and laptops, and 2) my camera gear and other items are packed in a single organized layer that's easily viewable by the TSO on the scanner.

Back when I did travel with a DSLR, I created an insert for a 20" rollaboard to organize my gear, turning the bag into a camera bag that doesn't scream "Camera Bag!". I still use that insert today, even though I don't travel with a DSLR any more. I find it helpful to organize my gear, and I have never had the bag pulled aside for a search.

This is what the insert looked like in 2011.