This is probably because you are extending the 
Register class, and not the 
RegisterForm service. You are also defining 2 classes in the same PHP file, which is wrong.
Change your extension definition to this:
	
	
	
		Code:
	
	
		Base class name: XF\Service\User\RegisterForm
Extension class name: SPAMINATOR\XF\Service\User\RegisterForm
	 
 
Then delete your previous PHP file.
Create a new PHP file: 
src/addons/SPAMINATOR/XF/Service/User/RegisterForm.php
	
	
	
		PHP:
	
	
		<?php
namespace SPAMINATOR\XF\Service\User;
class RegisterForm extends XFCP_RegisterForm
{
    protected $spaminatorHashedFields = [
        'accepts'
    ];
    protected $spaminatorHoneyPotFields = [
        'accepts_hp'
    ];
   
    public function __construct(\XF\App $app, \XF\Session\Session $session = null)
    {
        parent::__construct($app, $session);
       
        $this->hashedFields = array_merge($this->hashedFields, $this->spaminatorHashedFields);
        $this->honeyPotFields = array_merge($this->honeyPotFields, $this->spaminatorHoneyPotFields);
    }
}