Its not a bug - its a versioning hack to do with backwards compatibility in Unity.
Theres a version flag in there that tells the sliced sprite what version it is. It defaults to 0 for backwards compatibility, but it really needs to be 1 for normal use (which the Create > tk2d > Sliced Sprite menu does).
If you switch the inspector to debug mode, then turn off legacy mode and switch back, it should then work as expected. If creating from code, set legacyMode = false, and that will then work as expected.
In any case, there is a fixed automatic upgrade path in 2D Toolkit 2.0, and it should behave correctly in there.