Drag and drop from Visual Studio Toolbox to the designer area doesn't allow you to drop the corresponding control inside the content of the selected tab item. 

To work this around, add a Grid panel with transparent background inside the RadTabItem's in XAML. You can do this also  through the toolbox.

 <telerik:RadTabItem x:Name="RadTabItem1" Header="Tab item 1">
	<Grid Background="Transparent" />
 </telerik:RadTabItem>