Sub-Menu Doesn’t Display sub-levels on mobile devices

For page widths > 620px, 2nd level pages in Sub-Menu display either horizontally 620px < page width < 880px or vertically Page width > 880px. But they don’t display at all for page widths < 620px.

I see this in style.css for max-width: 620px (line 1465):
nav#subnav ul.menu li.menu-item ul.sub-menu li.menu-item {
display: none;
}

This appears to be a conscious decision to not display sub-level menu items but why? I want the navigation below the header image instead of above it, but the Sub-Menu becomes useless for smartphones.

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