Delete all terms when choosing "<none>" option

Using HEAD. Right now if i have a Vocabulary with 3 levels

Level A
-Level B
--Level C

1. I create a new node and tag with the deepest Level C term (saving all the path)
2. I get Level A > Level B > level C

1. I change this node and tag with Level B term leaving the Level C empty
2. I get Level A > Level B

1. I change this node and tag it with <none> in the first Level A
2. I get Level A > Level B

1. I change this node and tag it with Labelname in the first Level A
2. I get Level A > Level B

1. I change this node and tag it with Labelname in the Level B
2. I get Level A

its seems its impossible to get rid of the Level A tag.