I've found a strange behavior, something related with the cache, I guess.
When I'm in the checkout just give me the options:
And I'm pretty sure that "Checkout as guest" is enabled.
And when I navigate in private mode I can see the checkout as guest.
Checking the file: ../template/persistent/checkout/onepage/login.phtml and there is a condition
<?php if( $this->getQuote()->isAllowedGuestCheckout() ): ?>
The really weird thing is that when I'm in private mode (still with that xD) or in a browser with clear cache or in a browser that I've never used that condition is true, but other way is false.