Why does Stack always show the network names?

Because the name under the mark is what the style is — without it a Stack tile is a coloured square with nothing to read, which is what Marks only is for. So the Show network names menu is greyed while Stack is chosen, exactly as it is for Marks only, which never shows them. The one exception is the pinned bar on a phone: a 50px-wide tile has no room for "WhatsApp", so the bar shows marks only below 600px and goes back to the shorter height at the same moment.