Why do the ASP.NET Identity interfaces use strings for primary and foreign keys

I'm looking at the interfaces on the new ASP.NET Identity classes and the database it creates using Entity Framework Code First. I'm using the Visual Studio 2013 RC.

At first glance the database schema looks reasonably normal:

But all the keys are NVARCHAR(128)

And for some crazy reason AspNetUserSecrets.Id is a PK that looks like it could point to more than one record in the AspNetUsers table. Does this mean multiple AspNetUsers will have to share the same password?

When I look at the Looking at the interfaces you're forced to implement, these are all strings…

public class User : IUser
{
    public string Id { get; set; }
    public string UserName { get; set; }
}

public class UserSecret : IUserSecret
{
    public string UserName { get; set; }
    public string Secret { get; set; }
}

public class UserRole : IUserRole
{
    public string UserId { get; set; }
    public string RoleId { get; set; }
}

public class UserClaim : IUserClaim
{
    public string UserId { get; set; }
    public string ClaimType { get; set; }
    public string ClaimValue { get; set; }
}

public class UserManagement : IUserManagement
{
    public string UserId { get; set; }
    public bool DisableSignIn { get; set; }
    public DateTime LastSignInTimeUtc { get; set; }
}

public class Tokens : IToken
{
    public string Id { get; set; }
    public string Value { get; set; }
    public DateTime ValidUntilUtc { get; set; }
}

public class UserLogin : IUserLogin
{
    public string UserId { get; set; }
    public string LoginProvider { get; set; }
    public string ProviderKey { get; set; }
}

public class Role : IRole
{
    public string Id { get; set; }
    public string Name { get; set; }
}

So I'm coming to terms with the fact that I may have to implement this using strings for PK and FK relationships.

But I'd really love to know WHY it's built like this…?

EDIT: Time has passed and there are now articles on how to extend the asp.net identity to use int (or guid) fields:

http://www.asp.net/identity/overview/extensibility/change-primary-key-for-users-in-aspnet-identity