Because when you block someone on Facebook, you automatically unfriend them.How To Unblock Friends And People On Facebook
1) Go to your Facebook page;
2) Click the ‘Account’ tab;
3) Then select ‘Privacy Settings’;
4) You will see a new page, click on the ‘Edit list’ button;
5) When you refresh the page, you'll see a list of blocked friends and enemies;
6) Click on the ‘Unblock’ button.