Magento: Force Newsletter Subscription in registration

You need to edit the file /app/design/frontend/your_package/your_theme/template/customer/form/register.phtml.

or in Magento 1.9 app/design/frontend/rwd/default/template/persistent/customer/form/register.phtml.

1-Find and remove the small piece code:

<li class="control">
    <div class="input-box">
        <input type="checkbox" name="is_subscribed" title="<?php echo $this->__('Sign Up for Newsletter') ?>" value="1" id="is_subscribed"<?php if($this->getFormData()->getIsSubscribed()): ?> checked="checked"<?php endif; ?> class="checkbox" />
    </div>
    <label for="is_subscribed"><?php echo $this->__('Sign Up for Newsletter') ?></label>
</li>

2- Add the following code just after this:

<input type="hidden" name="success_url" value="<?php echo $this->getSuccessUrl() ?>" />
<input type="hidden" name="error_url" value="<?php echo $this->getErrorUrl() ?>" />
<input type="hidden" name="is_subscribed" value="1" id="is_subscribed"  />