Turtles are no different than most other living creatures on earth in that they are in fact able to walk backwards as well as forward. Obviously turtles are able to back-peddle while in water, however they are also able to walk backwards while on land. However it has been suggested that they may be unable to walk backwards when turning corners.
Something to consider is that although a turtle can walk backwards, they are unable to run backwards at high speeds. They also are also usually unable to flip themselves back over onto their feet if they happen to flip onto their backs accidently. Some turtles have died as a result of this as it leaves them vulnerable and makes their undersides “cook” if exposed in the sun for too long. Others have been known to starve to death while on their backs.
Usually a turtle will walk backwards to try and avoid a predator or to retreat back into its shell for protection when it feels threatened.