Why Does Membership Isapproved Set Itself to False?

The MembershipUser.IsApproved Property details will clear you doubt regarding this.

Gets or sets whether the membership user can be authenticated. As like some administrator is moderating the website and it is false if your account has not yet been approved by the site's administrator. If he/she approves then it will set to true.

for more information, you should go through Examining ASP.NET's Membership, Roles, and Profile - Part 4 By Scott Mitchell

You can approve user by code as:

Dim User As MembershipUser
User = Nothing
User = Membership.GetUser("MyUsername")
If (User Not Nothing)
then 
User.IsApproved = False 
End If

Ref:
Make New User default to IsApproved = true
Error approving membership user
Unlocking and Approving User Accounts (C#)

  protected void IsApproved_CheckedChanged(object sender, EventArgs e)
    {
         // Toggle the user's approved status
         string userName = Request.QueryString["user"];
         MembershipUser usr = Membership.GetUser(userName);
         usr.IsApproved = IsApproved.Checked;
         Membership.UpdateUser(usr);
         StatusMessage.Text = "The user's approved status has been updated.";
    }

Use MemberShip to manage this stuff. Hope this help..