It was just a design decision. A user's personal workspace is where new objects are placed by default, when you have the All workspace selected. This means that by default, objects you create are only accessible by you, so that you don't accidentally share an object. It can't be deleted because it's needed by the system to place those objects.
Another way we could have implemented it would be to allow objects not to have a workspace, but these objects would be visible by all users or none, because permissions are defined by workspaces (or we would have to also change how permissions work). And another way would be to allow setting the default workspace for a user. This one may be a better choice, but we think it's low priority to change it now. Can you think of another alternative?
We are currently designing OpenGoo 2.0, and it will see lots of changes on how objects are organized. This issue of where to place objects by default will be considered.