Well, it's how the foundation grid works. It's more grid specific setting rather then SXA one. But thanks to the fact that SXA supports different grid system there is an option to customize this.
Just navigate to your theme item and look for PlaceholderSettings field:
Change settings from Fixed to Flex for one of the placeholders or to all of them. Here I've just changed this to the Main placeholder and this gives me something like that:
By changing those settings you change the CSS class which is assigned to the certain placeholder. Those classes are defined per grid system. For the Foundation grid they are defined here:
/sitecore/system/Settings/Feature/Experience Accelerator/Foundation/Grid Definition: