Hi,
HikaShop does not include bootstrap css/js. Joomla 3.x includes it by default and templates done for Joomla 3.x are supposed to load them.
So when you activate that "use bootstrap design" option, HikaShop views are displayed with the HTML/classes that are compatible with bootstrap css/js so that you benefit from the bootstrap responsiveness, general layout design, etc.
It's always possible to have a plugin loading the bootstrap CSS/JS instead of the template if the template doesn't do it:
extensions.joomla.org/extensions/style-a...ign/templating/21856
That way, you can even use the bootstrap design of HikaShop on Joomla 2.5 with templates which don't include it.
So if you look for example at the "listing_div" view files via the menu Display->views, you will see several
if(HIKASHOP_RESPONSIVE) {
which will display some code or another if the option is activated or not. But as you can see, most of the code is common to both.