Pagerank distribution - Why ony main page is page rank 3 and category pages are 0 - SEO?

[teriff]

Hi there, there is one major thing about SEO in prestashop that I have been thinking of and that is page rank distribution...

For example my highest ranked store:
http://www.ji-style.se/ - pr3

On all other pages it's 0...

While shops like this one : http://www.asos.com/Woman/
Has pr 5 on main page, 4-3 on category, 1-0 on product and so on like a pyramid.

Someone know why prestashop is like this, and how much it does affect searches on level 2 keywords ( category keywords )
Ho much value main page gives to the category pages in googles eyes even if there is no page rank?