What causing WebMatrix.Data.dll and WebMatrix.WebData.dll to be added to my bin directory

They are added by NuGet packages that come with ASP.NET MVC 4 project templates.

You can get more information at What is the Microsoft ASP.NET Web Pages 2 Data Nuget Package for?

These assemblies provide Simple Membership Provider that is conflicting with your authentication. Are you using forms auth & have you correctly Implemented Authorize attribute? If your controllers are marked with Authorize attributes correctly then you will not have any problem. Presence of these dll is not the problem. Your controllers are not property authorized.