This behavior is expected. This allows for a legend to be customized so that the legend items can control the visibility of the series. For reference see this example http://demos.telerik.com/silverlight/#ChartView/LargeData. 
The legend items have been retemplated so that the legend item contains a toggle button, which controls the visibility of a series. In order to be able to make a series Visible, the toggle button has to be visible, which in turn means that the legend item has to be Visible. 
If you need to have the legend item hidden, when the series is not visible, you can use a custom legend item template. In the template you can bind its visibility to the Visibility of the series presenter: Visibility="{Binding Presenter.Visibility}". 
An alternative is to set the LegendSettings property to null when the Visibility is Collapsed, you can use a Converter for this purpose.