Because tortoises do. It's not unusual, there are various theories why they do it. Some believe that they do it in order to consume beneficial bacteria. Some believe that it's done due to a nutritional deficiency. Some believe that it's done for additional fiber or protein in their diet.
If it bothers you, try and remove the stool when you first see it. I am sooo glad I have not witnessed this from any of my tortoise...yet. :shy: hope I never do!!
Won't hurt them. Many animals eat their poo. Gross but perfectly natural. Some also do it as a defense technique to hide their presence from predators.
Others do it to be dominant (dogs eating another's poop).
I had my box turtle with me while pulling weeds. I missed some of my dogs poop while cleaning the yard and the boxie found it and thought it was a snack. Gross!
The only thing I would be concerned with is chemicals from a dog receiving heartworm meds, or the like. When they poop, some of those chemicals remain in the feces. If eaten by your tort, they could get sick. Also, there's the pinworms and other parasites that can transfer from poop to your tort.