If both Foo/FizzBuzz subweb and Foo.FizzBuzz topic exist at the same time, problems may occur. So you cannot create Foo/FizzBuz subweb if Foo.FizzBuzz topic exists and vice versa.
Until that conflict avoidance mechanism was introduced, you were able to create conflicting subwebs and conflicting topics. So you may have Foo/FizzBuzz subweb and FizzBuzz
topic coexist for historical reasons. To cope with that situation, the edit script allows editing a conflicting topic but the save script doesn't.
The save script must disallow to save a conflicting topic if it doesn't exist, but it shoudl allow to save it if it already exists.