One of the new features in Silverlight 4 is implicit styling (thank you, Microsoft!). To prevent unwanted surprises of inheritance there some limitations. It’s only possible to set the exact TargetType on a style that you wish to use. Hence it’s not possible to set TargetType to ButtonBase and let all types of buttons (Button, CheckBox, RadioButton…) implicit use the style.
http://www.silverlightshow.net/items/Implicit-Styles-in-Silverlight-4.aspx
by xamlgeek