The license terms need to be alittle more implicite.  Meaning that it says that it prohibits the removal of core attributes without permission but does not say anything about the index page itself.  The closest it comes to is using the term "front end" which for many means index but it is not implicite.

So here is what i am suggesting, that we modify the core in order to include the stand alone login page under that front end "core" unbrella. 

Here is why..

if someone makes the sign on page the index by simply changing the global permissions to no guest and moves the join page anywhere but the main menu, the signon page will become the main page by itself. And there is no attribute on that page which not only makes it difficult to offer proper support but also bypasses the attribute requirement.

My suggestion here is to modify the core

in ow_system_plugins/base/views/components/sign_in.html

on the bottom of that file you currently have

{/form}

{/div}

my suggestion is AFTER the close form {/form} but BEFORE the close div {/div} you add this

<span style="display:inline;">
{text key='base+copyright'}

<span style="float:right;">
{if $bottomPoweredByLink}
 {$bottomPoweredByLink}
{/if}
</span>
</span>       

 this will add the copyright as well as the attribute to that page, see image below.  

Note: I wrapped the bottomPoweredLink with an if just to help keep errors down but we dont have to do that we can just put the var itself as it is in the theme files, that is up to you if you want to wrap in IF statement or not.

I think this would be a good move in the right direction to help keep inline with the license requirements.

here is the image of the resulting code.

Thanks