October,
I would guess eating animal feces in the wild is an opportunistic encounter to get loads of calcium and other trace nutrients. Once the feces are gone its back to the usual grub of weeds. In your yard, the feces are much more common, and the tortoises probably don't have the instinct to only eat them on occasion. Probably, your tortoise would only eat the dog poop if he could.
Sort of like if we were cave men, the occasional bag of potato chips would be a valuable source of additional calories. Here in our time, we don't need those extra calories, but we don't have the sense to stop!
I think you're probably right about the actual risk of harm. Your dogs poop is probably similar to what a wild tortoise would find, and the majority of parasites that might live in a warm blooded canine are probably not going to survive in an ectotherm like your tortoise. As long as you or your tortoise limits his consumption to occasional, it MIGHT be safe, and perhaps very beneficial. The risk is that you don't know, and if your tortoise starts developing health problems, there's no way for you to know whether the dog poop caused the issue or if something else is responsible.
Oh speaking of parasites the human or beef tape worm is very common in cattle in the U.S. so I wouldn't give dogs or turtles any raw meat from cattle. But thats just me I like my cow well chared.