Squarespace is a 'hosted platform' – so you cannot use this website with any other ISP host.
That's just how it is.
The good news about that is that they take care of all of the basic management issues that arise constantly. Issues like cross-browser incompatability, or upgrades in CSS from 1 to 2 to 3, and HTML to HTML5 code etc.
Plus the themes are beautiful and they are really cool, nice guys and gals, based in New York, that really care about their end product – you!