Theme extend file not working from modules - why?

[Timpet]

The standard theme head.tpl file holds

{block name='head_seo'}
some code
{/block}

The smarty engine allows us to extend the file / block. This is done from product.tpl that inserts a canonical tag

But when i try to do the same from a modules .tpl it breaks and returns an error.

Why cant i make this work from a module .tpl when it works fine from the themes template files?