C# - Why DataClassesDataContext Has No Default Constructor

I have a connection string defined in my web.config like:

<connectionStrings>
<add name="LibraryConnectionString" connectionString="Server=.SQLEXPRESS3;Database=Library;Integrated Security=true" />

Well...lI do not understand why when i drag-and-drop tables into a new DataClasses dbml it does not construct a default constructor specifying the connection string. I only have constructor with params like:

public DataClassesDataContext(string connection) :
base(connection, mappingSource)
{
OnCreated();
}

I do need a default constructor for LinqDataSource..