Mosaic Tile Permissions

I'm using plone.app.mosaic and I have added a custom tile. Only editors should be able to add this tile to a layout. But if this tile is already on a layout everybody (contributors) should be able to edit this tile.

This is how I register the tile in zcml

  <plone:tile
    name="test.tiles.teaser.teasertile"
    title="A tile that shows Teaser"
    description=""
    add_permission="test.tiles.teaser.addteasertile"
    edit_permission="test.tiles.teaser.editteasertile"
    delete_permission="test.tiles.teaser.deleteteasertile"
    template="templates/teasertile.pt"
    class=".tiles.TeaserTile"
    permission="zope2.View"
    for="plone.dexterity.interfaces.IDexterityContent"
    layer="test.tiles.interfaces.ILayoutLayer"
    schema="test.tiles.tiles.ITeaserTileData"
    />

Now I would like to grant "test.tiles.teaser.addteasertile" permission to editors and "test.tiles.teaser.editteasertile" to editors and contributors. But contributors can not edit the tile. They get a permission denied message. If I grant "test.tiles.teaser.addteasertile" to contributors they can edit the tile - but also add this tile.

I am not sure if I understand zcmls "add_permission", "edit_permission" and "delete_permission" correctly. Can someone explain it in more detail? (Or is this a bug?)

[versions]
plone.app.mosaic = 1.0
plone.tiles = 1.6
plone.app.tiles = 2.2.1
plone.app.standardtiles = 1.0
plone.app.blocks = 3.1.0
plone.app.drafts = 1.0

"ZCML" has no intrinsic semantics for "{add|edit|delete}_permission". The handler behind "plone:tile" and (even more so) the components it registers decide how to interprete them.

From my point of view, this looks like a bug.